diff --git a/src/lib_data_encoding/binary.ml b/src/lib_data_encoding/binary.ml
index f937e7445..407054d4a 100644
--- a/src/lib_data_encoding/binary.ml
+++ b/src/lib_data_encoding/binary.ml
@@ -32,13 +32,13 @@ let rec length : type x. x t -> x -> int = fun e ->
   | Int32 -> fun _ -> Size.int32
   | Int64 -> fun _ -> Size.int64
   | RangedInt { minimum ; maximum } ->
-      fun _ -> integer_to_size @@ range_to_size ~minimum ~maximum
+      fun _ -> Size.(integer_to_size @@ range_to_size ~minimum ~maximum)
   | Float -> fun _ -> Size.float
   | RangedFloat _ -> fun _ -> Size.float
   | Bytes `Fixed n -> fun _ -> n
   | String `Fixed n -> fun _ -> n
   | String_enum (_, arr) ->
-      fun _ -> integer_to_size @@ enum_size arr
+      fun _ -> Size.(integer_to_size @@ enum_size arr)
   | Objs (`Fixed n, _, _) -> fun _ -> n
   | Tups (`Fixed n, _, _) -> fun _ -> n
   | Union (`Fixed n, _, _) -> fun _ -> n
@@ -52,7 +52,7 @@ let rec length : type x. x t -> x -> int = fun e ->
       let length2 = length e2 in
       fun (v1, v2) -> length1 v1 + length2 v2
   | Union (`Dynamic, sz, cases) ->
-      let tag_size = tag_size sz in
+      let tag_size = Size.tag_size sz in
       let case_length (Case { encoding = e ; proj }) =
         let length v = tag_size + length e v in
         fun v -> Option.map ~f:length (proj v) in
@@ -94,7 +94,7 @@ let rec length : type x. x t -> x -> int = fun e ->
         | [] -> (List.rev acc, json_only_cases)
         | Case { tag = Json_only } :: tl -> case_lengths true acc tl
         | Case { encoding = e ; proj ; tag = Tag _ } :: tl ->
-            let length v = tag_size sz + length e v in
+            let length v = Size.tag_size sz + length e v in
             case_lengths
               json_only_cases
               ((fun v ->
@@ -339,7 +339,7 @@ let rec write_rec
           if v < minimum || v > maximum
           then invalid_arg (Printf.sprintf "Integer %d not in range [%d, %d]." v minimum maximum) ;
           let v = if minimum >= 0 then v - minimum else v in
-          match range_to_size ~minimum ~maximum with
+          match Size.range_to_size ~minimum ~maximum with
           | `Uint8 -> uint8 v
           | `Uint16 -> uint16 v
           | `Uint30 -> uint30 v
@@ -362,7 +362,7 @@ let rec write_rec
   | String_enum (tbl, arr) ->
       (fun v ->
          let value = get_string_enum_case tbl v in
-         match enum_size arr with
+         match Size.enum_size arr with
          | `Uint30 -> uint30 value
          | `Uint16 -> uint16 value
          | `Uint8 -> uint8 value)
@@ -424,7 +424,7 @@ let rec write_rec_buffer
                             value minimum maximum) ;
         let value = if minimum >= 0 then value - minimum else value in
         begin
-          match range_to_size ~minimum ~maximum with
+          match Size.range_to_size ~minimum ~maximum with
           | `Uint30 -> uint30 value buffer
           | `Uint16 -> uint16 value buffer
           | `Uint8 -> uint8 value buffer
@@ -438,7 +438,7 @@ let rec write_rec_buffer
                             value minimum maximum) ;
         float value buffer
     | String_enum (tbl, arr) ->
-        (match enum_size arr with
+        (match Size.enum_size arr with
          | `Uint30 -> BufferedWriter.uint30
          | `Uint16 -> BufferedWriter.uint16
          | `Uint8 -> BufferedWriter.uint8)
@@ -638,7 +638,7 @@ module Reader = struct
         cases in
     fun buf ofs len ->
       let ofs, tag = read_tag sz buf ofs len in
-      try List.assoc tag read_cases buf ofs (len - tag_size sz)
+      try List.assoc tag read_cases buf ofs (len - Size.tag_size sz)
       with Not_found -> raise (Unexpected_tag tag)
 
 end
@@ -661,7 +661,7 @@ let rec read_rec : type a. a t-> MBytes.t -> int -> int -> int * a = fun e ->
   | RangedInt { minimum ; maximum } ->
       (fun buf ofs alpha ->
          let ofs, value =
-           match range_to_size ~minimum ~maximum with
+           match Size.range_to_size ~minimum ~maximum with
            | `Int8 -> int8 buf ofs alpha
            | `Int16 -> int16 buf ofs alpha
            | `Int31 -> int31 buf ofs alpha
@@ -686,7 +686,7 @@ let rec read_rec : type a. a t-> MBytes.t -> int -> int -> int * a = fun e ->
   | String_enum (_, arr) -> begin
       fun buf ofs a ->
         let ofs, ind =
-          match enum_size arr with
+          match Size.enum_size arr with
           | `Uint8 -> uint8 buf ofs a
           | `Uint16 -> uint16 buf ofs a
           | `Uint30 -> uint30 buf ofs a in
@@ -944,7 +944,7 @@ module Stream_reader = struct
         | Int64  -> next_path path (fst (int64  buf))
         | RangedInt { minimum ; maximum }  ->
             let (stream, ranged) =
-              match range_to_size ~minimum ~maximum with
+              match Size.range_to_size ~minimum ~maximum with
               | `Int8 -> int8 buf
               | `Int16 -> int16 buf
               | `Int31 -> int31 buf
@@ -973,7 +973,7 @@ module Stream_reader = struct
 
         | String_enum (_, arr) ->
             next_path path
-              (match enum_size arr with
+              (match Size.enum_size arr with
                | `Uint8 -> fst @@ uint8 buf
                | `Uint16 -> fst @@ uint16 buf
                | `Uint30 -> fst @@ uint30 buf)
@@ -1051,7 +1051,7 @@ module Stream_reader = struct
             in
             begin match opt with
               | None -> raise (Unexpected_tag ctag)
-              | Some func -> func (len - (tag_size sz))
+              | Some func -> func (len - (Size.tag_size sz))
             end
 
         | Dynamic_size e ->
diff --git a/src/lib_data_encoding/encoding.ml b/src/lib_data_encoding/encoding.ml
index 6bfe5747a..d51434c54 100644
--- a/src/lib_data_encoding/encoding.ml
+++ b/src/lib_data_encoding/encoding.ml
@@ -28,28 +28,6 @@ let apply ?(error=No_case_matched) fs v =
   loop fs
 
 
-module Size = struct
-  let bool = 1
-  let int8 = 1
-  let uint8 = 1
-  let char = 1
-  let int16 = 2
-  let uint16 = 2
-  let uint30 = 4
-  let uint32 = 4
-  let uint64 = 8
-  let int31 = 4
-  let int32 = 4
-  let int64 = 8
-  let float = 8
-end
-
-type tag_size = [ `Uint8 | `Uint16 ]
-
-let tag_size = function
-  | `Uint8 -> Size.uint8
-  | `Uint16 -> Size.uint16
-
 module Kind = struct
 
   type t =
@@ -99,7 +77,7 @@ module Kind = struct
     | [] -> assert false (* should be rejected by Data_encoding.union *)
     | k :: ks ->
         match List.fold_left merge k ks with
-        | `Fixed n -> `Fixed (n + tag_size sz)
+        | `Fixed n -> `Fixed (n + Size.tag_size sz)
         | k -> k
 
 end
@@ -131,7 +109,7 @@ type 'a desc =
   | Objs : Kind.t * 'a t * 'b t -> ('a * 'b) desc
   | Tup : 'a t -> 'a desc
   | Tups : Kind.t * 'a t * 'b t -> ('a * 'b) desc
-  | Union : Kind.t * tag_size * 'a case list -> 'a desc
+  | Union : Kind.t * Size.tag_size * 'a case list -> 'a desc
   | Mu : Kind.enum * string * ('a t -> 'a t) -> 'a desc
   | Conv :
       { proj : ('a -> 'b) ;
@@ -168,41 +146,6 @@ and 'a t = {
   mutable json_encoding: 'a Json_encoding.encoding option ;
 }
 
-type signed_integer = [ `Int31 | `Int16 | `Int8 ]
-type unsigned_integer = [ `Uint30 | `Uint16 | `Uint8 ]
-type integer = [ signed_integer | unsigned_integer ]
-
-let signed_range_to_size min max : [> signed_integer ] =
-  if min >= ~-128 && max <= 127
-  then `Int8
-  else if min >= ~-32_768 && max <= 32_767
-  then `Int16
-  else `Int31
-
-(* max should be centered at zero *)
-let unsigned_range_to_size max : [> unsigned_integer ] =
-  if max <= 255
-  then `Uint8
-  else if max <= 65535
-  then `Uint16
-  else `Uint30
-
-let integer_to_size = function
-  | `Int31 -> Size.int31
-  | `Int16 -> Size.int16
-  | `Int8 -> Size.int8
-  | `Uint30 -> Size.uint30
-  | `Uint16 -> Size.uint16
-  | `Uint8 -> Size.uint8
-
-let range_to_size ~minimum ~maximum : integer =
-  if minimum < 0
-  then signed_range_to_size minimum maximum
-  else unsigned_range_to_size (maximum - minimum)
-
-let enum_size arr =
-  unsigned_range_to_size (Array.length arr)
-
 type 'a encoding = 'a t
 
 let rec classify : type a. a t -> Kind.t = fun e ->
@@ -220,14 +163,14 @@ let rec classify : type a. a t -> Kind.t = fun e ->
   | Int32 -> `Fixed Size.int32
   | Int64 -> `Fixed Size.int64
   | RangedInt { minimum ; maximum } ->
-      `Fixed (integer_to_size @@ range_to_size ~minimum ~maximum)
+      `Fixed Size.(integer_to_size @@ range_to_size ~minimum ~maximum)
   | Float -> `Fixed Size.float
   | RangedFloat _ -> `Fixed Size.float
   (* Tagged *)
   | Bytes kind -> (kind :> Kind.t)
   | String kind -> (kind :> Kind.t)
   | String_enum (_, cases) ->
-      `Fixed (integer_to_size (enum_size cases))
+      `Fixed Size.(integer_to_size @@ enum_size cases)
   | Obj (Opt (kind, _, _)) -> (kind :> Kind.t)
   | Objs (kind, _, _) -> kind
   | Tups (kind, _, _) -> kind
diff --git a/src/lib_data_encoding/encoding.mli b/src/lib_data_encoding/encoding.mli
index 5cf24fbe6..f9364fc64 100644
--- a/src/lib_data_encoding/encoding.mli
+++ b/src/lib_data_encoding/encoding.mli
@@ -50,26 +50,6 @@
  * predicates, etc.) *)
 (* TODO: move the doc into the packing module *)
 
-module Size: sig
-  val bool: int
-  val int8: int
-  val uint8: int
-  val char: int
-  val int16: int
-  val uint16: int
-  val uint30: int
-  val uint32: int
-  val uint64: int
-  val int31: int
-  val int32: int
-  val int64: int
-  val float: int
-end
-
-type tag_size = [ `Uint8 | `Uint16 ]
-
-val tag_size: tag_size -> int
-
 val apply: ?error:exn -> ('a -> 'b option) list -> 'a -> 'b
 
 module Kind: sig
@@ -84,7 +64,7 @@ module Kind: sig
 
   val merge : t -> t -> t
 
-  val merge_list: tag_size -> t list -> t
+  val merge_list: Size.tag_size -> t list -> t
 
 end
 
@@ -115,7 +95,7 @@ type 'a desc =
   | Objs : Kind.t * 'a t * 'b t -> ('a * 'b) desc
   | Tup : 'a t -> 'a desc
   | Tups : Kind.t * 'a t * 'b t -> ('a * 'b) desc
-  | Union : Kind.t * tag_size * 'a case list -> 'a desc
+  | Union : Kind.t * Size.tag_size * 'a case list -> 'a desc
   | Mu : Kind.enum * string * ('a t -> 'a t) -> 'a desc
   | Conv :
       { proj : ('a -> 'b) ;
@@ -155,13 +135,6 @@ type 'a encoding = 'a t
 
 val make: ?json_encoding: 'a Json_encoding.encoding -> 'a desc -> 'a t
 
-type signed_integer = [ `Int31 | `Int16 | `Int8 ]
-type unsigned_integer = [ `Uint30 | `Uint16 | `Uint8 ]
-type integer = [ signed_integer | unsigned_integer ]
-val integer_to_size: integer -> int
-val range_to_size: minimum:int -> maximum:int -> integer
-val enum_size: 'a array -> [> unsigned_integer ]
-
 
 exception No_case_matched
 exception Unexpected_tag of int
diff --git a/src/lib_data_encoding/size.ml b/src/lib_data_encoding/size.ml
new file mode 100644
index 000000000..0109ebd40
--- /dev/null
+++ b/src/lib_data_encoding/size.ml
@@ -0,0 +1,65 @@
+(**************************************************************************)
+(*                                                                        *)
+(*    Copyright (c) 2014 - 2018.                                          *)
+(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
+(*                                                                        *)
+(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
+(*                                                                        *)
+(**************************************************************************)
+
+let bool = 1
+let int8 = 1
+let uint8 = 1
+let char = 1
+let int16 = 2
+let uint16 = 2
+let uint30 = 4
+let uint32 = 4
+let uint64 = 8
+let int31 = 4
+let int32 = 4
+let int64 = 8
+let float = 8
+
+type tag_size = [ `Uint8 | `Uint16 ]
+
+let tag_size = function
+  | `Uint8 -> uint8
+  | `Uint16 -> uint16
+
+
+type signed_integer = [ `Int31 | `Int16 | `Int8 ]
+type unsigned_integer = [ `Uint30 | `Uint16 | `Uint8 ]
+type integer = [ signed_integer | unsigned_integer ]
+
+let signed_range_to_size min max : [> signed_integer ] =
+  if min >= ~-128 && max <= 127
+  then `Int8
+  else if min >= ~-32_768 && max <= 32_767
+  then `Int16
+  else `Int31
+
+(* max should be centered at zero *)
+let unsigned_range_to_size max : [> unsigned_integer ] =
+  if max <= 255
+  then `Uint8
+  else if max <= 65535
+  then `Uint16
+  else `Uint30
+
+let integer_to_size = function
+  | `Int31 -> int31
+  | `Int16 -> int16
+  | `Int8 -> int8
+  | `Uint30 -> uint30
+  | `Uint16 -> uint16
+  | `Uint8 -> uint8
+
+let range_to_size ~minimum ~maximum : integer =
+  if minimum < 0
+  then signed_range_to_size minimum maximum
+  else unsigned_range_to_size (maximum - minimum)
+
+let enum_size arr =
+  unsigned_range_to_size (Array.length arr)
+
diff --git a/src/lib_data_encoding/size.mli b/src/lib_data_encoding/size.mli
new file mode 100644
index 000000000..65ab9f625
--- /dev/null
+++ b/src/lib_data_encoding/size.mli
@@ -0,0 +1,34 @@
+(**************************************************************************)
+(*                                                                        *)
+(*    Copyright (c) 2014 - 2018.                                          *)
+(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
+(*                                                                        *)
+(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
+(*                                                                        *)
+(**************************************************************************)
+
+val bool: int
+val int8: int
+val uint8: int
+val char: int
+val int16: int
+val uint16: int
+val uint30: int
+val uint32: int
+val uint64: int
+val int31: int
+val int32: int
+val int64: int
+val float: int
+
+type tag_size = [ `Uint8 | `Uint16 ]
+
+val tag_size: tag_size -> int
+
+type signed_integer = [ `Int31 | `Int16 | `Int8 ]
+type unsigned_integer = [ `Uint30 | `Uint16 | `Uint8 ]
+type integer = [ signed_integer | unsigned_integer ]
+val integer_to_size: integer -> int
+val range_to_size: minimum:int -> maximum:int -> integer
+val enum_size: 'a array -> [> unsigned_integer ]
+