diff --git a/spec/API_specification/dataframe_api/groupby_object.py b/spec/API_specification/dataframe_api/groupby_object.py
index a00cc9ec..43596a66 100644
--- a/spec/API_specification/dataframe_api/groupby_object.py
+++ b/spec/API_specification/dataframe_api/groupby_object.py
@@ -34,3 +34,6 @@ def std(self, skipna: bool = True) -> "DataFrame":
 
     def var(self, skipna: bool = True) -> "DataFrame":
         ...
+
+    def size(self) -> "DataFrame":
+        ...