This can be a surprisingly difficult thing to measure, for a little surprising reason:

The city with most floor space per person is among the cities with the lowest population density. It will be a city where the maximum amount of homes are detached houses. And probably one where the "urban" planning is less dense than in your standard suburbia.

It will be a very small city by population.

And that leads us to the problem: how do we define "city"?

There are several competing definitions for what constitutes a city, and since we're talking about cities at the very lowest end of the population scale, you'll get a big pile of different answers depending on what counts as a city fir whom.