diff --git a/pytorch_lightning/core/datamodule.py b/pytorch_lightning/core/datamodule.py
index 770745b322..e2ba05eee3 100644
--- a/pytorch_lightning/core/datamodule.py
+++ b/pytorch_lightning/core/datamodule.py
@@ -184,7 +184,7 @@ class LightningDataModule(CheckpointHooks, DataHooks, HyperparametersMixin):
         rank_zero_deprecation("DataModule property `dims` was deprecated in v1.5 and will be removed in v1.7.")
         self._dims = d
 
-    def size(self, dim=None) -> Union[Tuple, int]:
+    def size(self, dim=None) -> Union[Tuple, List[Tuple]]:
         """
         Return the dimension of each input either as a tuple or list of tuples. You can index this
         just as you would with a torch tensor.