To truly provide good advice I feel I need more information, it depends on a lot of factors.. what kind of birds, how many you want to keep.. inside vs. outside space of course.
I have SEVERAL coop pages here on BYC (Small, medium & large) just click View my BYC page under my avatar on the left.
I, like the previous poster, prefer to give as much room as possible and try to stick to 10sf each. Also because my chickens have to take turns ranging in the yard so not every coop gets out everyday... just a couple times per week. My designated breeding pens are 67sf and generally hold no more than 4 birds but can go up to 6 if needed. These are large Orpingtons