How large can a city be and still be regarded as a "college town"?
Consider metro area population. I know Cincinnati had over 2 million people and is certainly not a college town as I think of it. Columbus? Nope.
I'd probably limit it to about 500,000 regular residents as a stretch. I looked up Athens, GA, which I do consider to be a college town, 220,000 metro area, much larger than I thought.