You will get lots of different opinions on this question. In my personal opinion, I feel large fowl need at least 10 square feet per bird and bantam need 4 sq feet per bird. This can vary depending on breed - some are more pecky than others, some do really well in close quarters, etc. It also depends on your climate. If there is any chance your birds may be stuck in their coop (without access to the outdoors) due to heavy snowfall, extreme conditions, predator issue, etc ...you need to account for more inside space. Even if this is only for a day or two. Too little space is very stressful on birds and can lead to numerous problems in your management program.
Jody