fix: test size method

Author: johanneskoester

snakemake_interface_storage_plugins/tests.py

@@ -96,6 +96,7 @@ def _test_storage(self, tmp_path, directory=False):
             assert obj.exists()
 
             assert isinstance(obj.mtime(), (float, int))
+            assert isinstance(obj.size(), int)
 
             self._test_inventory(obj)