[Jtsummers]

Possibly with numbers. Can you properly visualize a tennis ball next to a baseball and tell which is larger? I was never much of an athlete (in the sports ball sense, at least) and have insufficient experience with either to tell you definitively which is larger (I looked it up, my guess was baseball and that turned out to be correct). The fact is that a small but still standard sized baseball is only 5mm larger than a large but still standard sized tennis ball. That is near enough that my memory (I played baseball last as a kid) could not distinguish between the size of the two objects.

But now I know the fact, a standard baseball is larger than a standard tennis ball (now, don't ask me in a few weeks what the actual sizes are, I will probably forget, though I will probably remember that the baseball is larger).

Another way is just having knowledge of relative sizes and the ability to perform rudimentary logic. A tennis ball feels about the same in my hand as a door knob (in terms of size and ability to wrap my hand around it, bigger than most but not by too much). A keyhole fits into a doorknob. I'd have to be really drunk to not realize the logical implication of that: A tennis ball won't fit into a keyhole (assuming both are standard sized and we're not talking about a comedian's prop key and keyhole).