When we recognize an object, do we automatically know how big it is in the world? We employed a Stroop-like paradigm, in which two familiar objects were presented at different visual sizes on the screen. Observers were faster to indicate which was bigger or smaller on the screen when the real-world size of the objects was congruent with the visual size than when it was incongruent--demonstrating a familiar-size Stroop effect. Critically, the real-world size of the objects was irrelevant for the task. This Stroop effect was also present when only one item was present at a congruent or incongruent visual size on the display. In contrast, no Stroop effect was observed for participants who simply learned a rule to categorize novel objects as big or small. These results show that people access the familiar size of objects without the intention of doing so, demonstrating that real-world size is an automatic property of object representation. (Contains 4 figures.)