It depends upon the density of that city, but here's a graphic depiction:
That's just about fitting everyone in a space, not supplying them with water, food, and everything else they'd need.
I ran across that graphic as the result of a search I did after reading the comments to an NPR article, "How China's One-Child Policy Led To Forced Abortions, 30 Million Bachelors." Someone in the comments had said: "The entire world population could theoretically live in Texas. Underpopulation in industrialized nations is the real problem." I thought the necessary space was quite a bit less than Texas. And somebody else said: "No, it 'theoretically' could not. The reason for that is that you have to be able to actually move. And you have to be able to move to the LAND that you are FARMING to grow food. The argument that you could pack people like sardines someplace is completely without any kind of logical merit."