diff --git a/doc/stringio/size.rdoc b/doc/stringio/size.rdoc
new file mode 100644
index 0000000..9323adf
--- /dev/null
+++ b/doc/stringio/size.rdoc
@@ -0,0 +1,5 @@
+Returns the number of bytes in the string in +self+:
+
+  StringIO.new('hello').size     # => 5  # Five 1-byte characters.
+  StringIO.new('тест').size      # => 8  # Four 2-byte characters.
+  StringIO.new('こんにちは').size # => 15 # Five 3-byte characters.
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 874a3a1..de09380 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1998,10 +1998,10 @@ strio_syswrite_nonblock(int argc, VALUE *argv, VALUE self)
 
 /*
  * call-seq:
- *   strio.length -> integer
- *   strio.size   -> integer
+ *   size -> integer
+ *
+ * :include: stringio/size.rdoc
  *
- * Returns the size of the buffer string.
  */
 static VALUE
 strio_size(VALUE self)