diff --git a/containerregistry/client/v2_2/append_.py b/containerregistry/client/v2_2/append_.py
index e3b0160e..021920d0 100755
--- a/containerregistry/client/v2_2/append_.py
+++ b/containerregistry/client/v2_2/append_.py
@@ -82,6 +82,7 @@ def __init__(self,
     self._config_file = json.dumps(config_file, sort_keys=True)
     manifest['config']['digest'] = docker_digest.SHA256(
         self._config_file.encode('utf8'))
+    manifest['config']['size'] = len(self._config_file)
     self._manifest = json.dumps(manifest, sort_keys=True)
 
   def manifest(self):