Lands around a metropolis

[Desdichado]

I'm asking because I'm putting together a world where there aren't many people left, and where those people congregate in big cities, because the world has become a very dangerous place. I'm trying to figure out how far around the city walls would the city need to control in order to be able to feed every one within the walls.

Are you talking about walled cities, surrounded by farmland, which are in turn further surrounded by far-flung walls? That's kinda an interesting idea.