Hmmm...the "biggest"...I'm going to say 1 million. As someone noted above, Rome and Alexandria has a million.
My world, I will say, has a single megapolis of that size. The second/next largest would be around 500,000. Those are the two giant, largest cities, both known the continent over. Often looked at as "sister cities" one sits on the southeastern coast and on a western sea coast they are referred to as, "the Gem of the East" and "the Jewel of the West," respectively.
Following those two, "Metropolis" cities, which I would put 50-100k, would number...six? Maybe seven.
"Cities" 25-50k... I'm thinking only about 5.
"Large Towns" or Medium towns with something notable, say 10-25k. Maybe a dozen.
Medium towns or smaller, farming villages, hamlets, trading posts, clumps of homesteads, lone "keeps" housing a few dozen to a few hundred, etc... there are 4 or 5 I can name off the top of my head ("starting points for most campaigns). But realistically, they are innumerable/generated as needed.
I guess that sounds like a lot, but if you consider we're talking about a continent of seven different established HUMAN nations, at least four nations of other species, and wide swathes of unconquered (or conquered, fallen, and reclaimed) wilderness. So, divvied up, it's really not that many people in any given place.