diff --git a/snakemake_interface_storage_plugins/tests.py b/snakemake_interface_storage_plugins/tests.py
index a3b548d..040b5f5 100644
--- a/snakemake_interface_storage_plugins/tests.py
+++ b/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) and obj.size() >= 0
 
             self._test_inventory(obj)