[spijdar]

> without ever being told one is smaller than the other and without being able to visualize both?

As opposed to never having seen or been told about a tennis ball or a keyhole, and expected to know which is smaller?

The idea of aphantasia doesn't imply being blind. Assuming someone is sighted, you can still see objects and internalize an order of "small to large" things without necessarily using mental imagery to make those connections. I'm not sure if I have some "kind of aphantasia" but I also don't generally visualize things in my head, instead remembering logical relations based on my (non-visual) memories.