I live in a city that is geographically close to the size of waterdeep, we have a ton of 2-4 story buildings & an estimated population of ~38k. Yea most of those buildings have yards, but postage stamp lots are the norm not the exception. If you level the solar farm, (freight) train yard, & various indistrial areas you might be able to double that... but doing that gets into waterdeep's other problem, it doesn't really have any industries that need an expensive city to economically function & doesn't have the sort of bureaucratic overhead needed to run an empire like ancient rome had.Population is literally half that in most sources.
I live in a city of 120k. Population density was more of course in older time but not that much more.
@Demetrios1453 200k would put waterdeep at a quarter the population of current day san francisco(884k) in just under a tenth the landmass (6.5 square miles & 46.8) square miles without all the modern & semimodern high rise apartment buildings sending it back towards Manilla type densities.