diff --git a/core/CCIO.ml b/core/CCIO.ml
index dae3d656..61f64cbb 100644
--- a/core/CCIO.ml
+++ b/core/CCIO.ml
@@ -116,8 +116,8 @@ let read_lines_l ic =
   with End_of_file ->
     List.rev !l
 
-let read_all ic =
-  let buf = ref (Bytes.create 256) in
+let read_all ?(size=1024) ic =
+  let buf = ref (Bytes.create size) in
   let len = ref 0 in
   try
     while true do
diff --git a/core/CCIO.mli b/core/CCIO.mli
index 80699140..73c1529e 100644
--- a/core/CCIO.mli
+++ b/core/CCIO.mli
@@ -83,8 +83,9 @@ val read_lines : in_channel -> string gen
 val read_lines_l : in_channel -> string list
 (** Read all lines into a list *)
 
-val read_all : in_channel -> string
-(** Read the whole channel into a buffer, then converted into a string *)
+val read_all : ?size:int -> in_channel -> string
+(** Read the whole channel into a buffer, then converted into a string.
+    @param size the internal buffer size @since NEXT_RELEASE *)
 
 (** {6 Output} *)