diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,10 @@
 # Changelog for wai-extra
 
+## 3.0.26.1
+
+* When available, supply the response size to custom loggers
+  [#757](https://github.com/yesodweb/wai/pull/757)
+
 ## 3.0.26
 
 * Throw 413 for too large payload
diff --git a/Network/Wai/Middleware/RequestLogger.hs b/Network/Wai/Middleware/RequestLogger.hs
--- a/Network/Wai/Middleware/RequestLogger.hs
+++ b/Network/Wai/Middleware/RequestLogger.hs
@@ -118,8 +118,8 @@
 customMiddleware :: Callback -> IO ZonedDate -> OutputFormatter -> Middleware
 customMiddleware cb getdate formatter app req sendResponse = app req $ \res -> do
     date <- liftIO getdate
-    -- We use Nothing for the response size since we generally don't know it
-    liftIO $ cb $ formatter date req (responseStatus res) Nothing
+    let msize = contentLength (responseHeaders res)
+    liftIO $ cb $ formatter date req (responseStatus res) msize
     sendResponse res
 
 customMiddlewareWithDetails :: Callback -> IO ZonedDate -> OutputFormatterWithDetails -> Middleware
@@ -129,12 +129,12 @@
   app req' $ \res -> do
     t1 <- getCurrentTime
     date <- liftIO getdate
-    -- We use Nothing for the response size since we generally don't know it
+    let msize = contentLength (responseHeaders res)
     builderIO <- newIORef $ B.byteString ""
     res' <- recordChunks builderIO res
     rspRcv <- sendResponse res'
     _ <- liftIO . cb .
-      formatter date req' (responseStatus res') Nothing (t1 `diffUTCTime` t0) reqBody =<<
+      formatter date req' (responseStatus res') msize (t1 `diffUTCTime` t0) reqBody =<<
       readIORef builderIO
     return rspRcv
 
diff --git a/wai-extra.cabal b/wai-extra.cabal
--- a/wai-extra.cabal
+++ b/wai-extra.cabal
@@ -1,5 +1,5 @@
 Name:                wai-extra
-Version:             3.0.26
+Version:             3.0.26.1
 Synopsis:            Provides some basic WAI handlers and middleware.
 description:
   Provides basic WAI handler and middleware functionality:
@@ -87,7 +87,7 @@
   default:            False
 
 Library
-  Build-Depends:     base                      >= 4.6 && < 5
+  Build-Depends:     base                      >= 4.8 && < 5
                    , bytestring                >= 0.10.4
                    , wai                       >= 3.0.3.0  && < 3.3
                    , old-locale                >= 1.0.0.2  && < 1.1