diff --git a/CHANGELOG.md b/CHANGELOG.md
index b55afc3b..405c3766 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,19 @@
+Version 59:
+
+* Integrated Beast INTERFACE (cmake)
+
+API Changes:
+
+* Change Body::size signature (API Change):
+
+Actions Required:
+
+* For any user-defined models of Body, change the function signature
+  to accept `value_type const&` and modify the function definition
+  accordingly.
+
+--------------------------------------------------------------------------------
+
 Version 58:
 
 * Fix unaligned reads in utf8-checker
diff --git a/doc/5_09_custom_body.qbk b/doc/5_09_custom_body.qbk
index 6f7294ed..2872e1ef 100644
--- a/doc/5_09_custom_body.qbk
+++ b/doc/5_09_custom_body.qbk
@@ -66,6 +66,11 @@ struct Body
     class reader;
 
     class writer;
+
+    // Optional member
+    static
+    std::uint64_t
+    size(value_type const&);
 };
 ```
 
diff --git a/doc/concept/Body.qbk b/doc/concept/Body.qbk
index 0214dffa..8a334ecb 100644
--- a/doc/concept/Body.qbk
+++ b/doc/concept/Body.qbk
@@ -48,10 +48,10 @@ In this table:
         the message body for serialization.
     ]
 ][
-    [`X::size(m)`]
+    [`X::size(X::value_type const& v)`]
     [`std::uint64_t`]
     [
-        If present, returns the serialized size of `m.body` not including
+        If present, returns the serialized size of `v` not including
         any chunked transfer encoding. When this function is provided,
         [link beast.ref.beast__http__message.prepare `message::prepare`]
         will automatically set the content length field based on the
@@ -80,10 +80,9 @@ struct body
     class writer;
 
     /// Returns the body's payload size
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(message<isRequest, empty_body, Fields> const& m);
+    size(value_type const& v);
 }
 ```
 
diff --git a/example/http-server/file_body.hpp b/example/http-server/file_body.hpp
index 512381f6..5faf75b5 100644
--- a/example/http-server/file_body.hpp
+++ b/example/http-server/file_body.hpp
@@ -48,10 +48,9 @@ struct file_body
 
         @return The size of the file in bytes.
     */
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(message<isRequest, file_body, Fields> const& m);
+    size(value_type const& v);
 
     /** Algorithm for retrieving buffers when serializing.
 
@@ -72,12 +71,11 @@ struct file_body
 
 //[example_http_file_body_2
 
-template<bool isRequest, class Fields>
 std::uint64_t
 file_body::
-size(message<isRequest, file_body, Fields> const& m)
+size(value_type const& v)
 {
-    return boost::filesystem::file_size(m.body);
+    return boost::filesystem::file_size(v);
 }
 
 //]
diff --git a/include/beast/http/detail/type_traits.hpp b/include/beast/http/detail/type_traits.hpp
index 4479f2ae..ea2e2284 100644
--- a/include/beast/http/detail/type_traits.hpp
+++ b/include/beast/http/detail/type_traits.hpp
@@ -62,17 +62,17 @@ struct has_value_type<T, beast::detail::void_t<
 /** Determine if a @b Body type has a size
 
     This metafunction is equivalent to `std::true_type` if
-    Body contains a static member function called `content_lengeth`.
+    Body contains a static member function called `size`.
 */
-template<class T, class M, class = void>
+template<class T, class = void>
 struct is_body_sized : std::false_type {};
 
-template<class T, class M>
-struct is_body_sized<T, M, beast::detail::void_t<
+template<class T>
+struct is_body_sized<T, beast::detail::void_t<
     typename T::value_type,
         decltype(
     std::declval<std::uint64_t&>() =
-        T::size(std::declval<M const&>()),
+        T::size(std::declval<typename T::value_type const&>()),
     (void)0)>> : std::true_type {};
 
 } // detail
diff --git a/include/beast/http/dynamic_body.hpp b/include/beast/http/dynamic_body.hpp
index 7397bc30..e6bb94f4 100644
--- a/include/beast/http/dynamic_body.hpp
+++ b/include/beast/http/dynamic_body.hpp
@@ -29,13 +29,11 @@ struct basic_dynamic_body
     using value_type = DynamicBuffer;
 
     /// Returns the content length of this body in a message.
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(message<isRequest,
-        basic_dynamic_body, Fields> const& m)
+    size(value_type const& v)
     {
-        return m.body.size();
+        return v.size();
     }
 
 #if BEAST_DOXYGEN
diff --git a/include/beast/http/empty_body.hpp b/include/beast/http/empty_body.hpp
index 1a563c60..59b31d5d 100644
--- a/include/beast/http/empty_body.hpp
+++ b/include/beast/http/empty_body.hpp
@@ -36,10 +36,9 @@ struct empty_body
     };
 
     /// Returns the content length of the body in a message.
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(message<isRequest, empty_body, Fields> const& m)
+    size(empty_body)
     {
         return 0;
     }
diff --git a/include/beast/http/impl/message.ipp b/include/beast/http/impl/message.ipp
index 45f6774f..45d9cf8c 100644
--- a/include/beast/http/impl/message.ipp
+++ b/include/beast/http/impl/message.ipp
@@ -285,8 +285,7 @@ size() const
     static_assert(is_body_reader<Body>::value,
         "BodyReader requirements not met");
 
-    return size(detail::is_body_sized<
-        Body, decltype(*this)>{});
+    return size(detail::is_body_sized<Body>{});
 }
 
 template<bool isRequest, class Body, class Fields>
diff --git a/include/beast/http/message.hpp b/include/beast/http/message.hpp
index f9808dd8..fdf77359 100644
--- a/include/beast/http/message.hpp
+++ b/include/beast/http/message.hpp
@@ -558,7 +558,7 @@ private:
     boost::optional<std::uint64_t>
     size(std::true_type) const
     {
-        return Body::size(*this);
+        return Body::size(body);
     }
 
     boost::optional<std::uint64_t>
diff --git a/include/beast/http/string_body.hpp b/include/beast/http/string_body.hpp
index 6bb357e8..eaccc9dc 100644
--- a/include/beast/http/string_body.hpp
+++ b/include/beast/http/string_body.hpp
@@ -31,13 +31,11 @@ struct string_body
     using value_type = std::string;
 
     /// Returns the content length of the body in a message.
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(
-        message<isRequest, string_body, Fields> const& m)
+    size(value_type const& v)
     {
-        return m.body.size();
+        return v.size();
     }
 
 #if BEAST_DOXYGEN
diff --git a/include/beast/http/string_view_body.hpp b/include/beast/http/string_view_body.hpp
index 06b3d69a..6a7df6b2 100644
--- a/include/beast/http/string_view_body.hpp
+++ b/include/beast/http/string_view_body.hpp
@@ -32,13 +32,11 @@ struct string_view_body
     using value_type = string_view;
 
     /// Returns the content length of this body in a message.
-    template<bool isRequest, class Fields>
     static
     std::uint64_t
-    size(message<isRequest,
-        string_view_body, Fields> const& m)
+    size(value_type const& v)
     {
-        return m.body.size();
+        return v.size();
     }
 
 #if BEAST_DOXYGEN