<p>Though visual illusions are widely studied in both philosophy and science, important aspects of them are not yet understood. One gap in our knowledge comes from size illusions such as the Ebbinghaus–Titchener and Ponzo illusions, since it’s not clear which type of size is misperceived in them: (1) angular size, the amount of visual field that an object appears in, (2) absolute size, an object’s actual physical dimensions, or (3) relative size, the object’s size as compared to a different object. This article investigates this issue. In it, I first provide a visual demonstration showing that the Ebbinghaus–Titchener illusion doesn’t include an angular size illusion (Option 1), then show how this might be used to infer that it also doesn’t include an absolute or relative size illusion (Options 2 and 3). The article draws the surprising conclusion that a size illusion might not involve any misperceptions of size but rather arises from mistaken beliefs about the content of our own visual experiences. Something else that the investigation reveals is that it appears as if our capacity to introspect on size illusions is limited in one or more ways, raising questions about its reliability as a tool for studying size illusions.</p>