Average amount of space an apartment building takes up?

I'm in the process of designing a fictional city and want to be able to get a rough population estimate.
I'm not counting the amount of floors, I just need the the total area for example: 46 by 88 feet.
I would also greatly appreciate knowing how many people on average live in an apartment building.
If you aren't good at math like me I would also appreciate links to some online calculators I could use.

EDIT: I tried doing the math and calculation but arrived at a ridiculously low number, the area of the city is 4% of Mars's surface area and all the farms are underground, the city is packed fairly tight too. I want to know what the population would be.