diff --git a/CHANGELOG.md b/CHANGELOG.md
index f575d215..9f9e3759 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@ Version 72:
 HTTP:
 
 * Tidy up set payload in http-server-fast
+* Refine Body::size specification
 
 --------------------------------------------------------------------------------
 
diff --git a/doc/concept/Body.qbk b/doc/concept/Body.qbk
index 87dd05c0..85cf5736 100644
--- a/doc/concept/Body.qbk
+++ b/doc/concept/Body.qbk
@@ -48,14 +48,37 @@ In this table:
         the message body for serialization.
     ]
 ][
-    [`X::size(X::value_type const& v)`]
+    [`X::size(X::value_type v)`]
     [`std::uint64_t`]
     [
-        If present, returns the serialized size of `v` not including
-        any chunked transfer encoding. When this function is provided,
+        This static member function is optional. It returns the payload
+        size of `v` not including any chunked transfer encoding. The
+        function shall not exit via an exception.
+
+        When this function is present:
+
+        * The function shall not fail
+
+        * A call to
+        [link beast.ref.beast__http__message.payload_size `message::payload_size`]
+        will return the same value as `size`.
+
+        * A call to
         [link beast.ref.beast__http__message.prepare_payload `message::prepare_payload`]
-        will automatically set the content length field based on the
-        value. Otherwise, the chunked transfer encoding will be set.
+        will remove "chunked" from the Transfer-Encoding field if it appears
+        as the last encoding, and will set the Content-Length field to the
+        returned value.
+
+        Otherwise, when the function is omitted:
+
+        * A call to
+        [link beast.ref.beast__http__message.payload_size `message::payload_size`]
+        will return `boost::none`.
+
+        * A call to
+        [link beast.ref.beast__http__message.prepare_payload `message::prepare_payload`]
+        will erase the Content-Length field, and add "chunked" as the last
+        encoding in the Transfer-Encoding field if it is not already present.
     ]
 ][
     [`is_body<X>`]
diff --git a/test/exemplars.cpp b/test/exemplars.cpp
index 11f728e3..d92dd56e 100644
--- a/test/exemplars.cpp
+++ b/test/exemplars.cpp
@@ -134,6 +134,8 @@ struct BodyWriter
     {
         // The specification requires this to indicate "no error"
         ec = {};
+
+        return boost::asio::buffer_size(buffers);
     }
 
     /** Called when the body is complete.