In general. If you have small bantams, in a mild climate and let the chickens free range daily, you don't need so much space.Yes, I considered that to fall under the category of boredom, as in stuck in a small pen without anything to do, but that's worth mentioning on its own as well. Bare minimum 4 square feet per bird inside the coop and 10 square feet per bird outside of the coop.
If you have a couple of roosters, very active chickens, and big breeds this is probably not enough. And if you have long and cold winters you certainly need more coop space.