Indoors my girls have 4sqft/bird, 4 are bantams, 2 are standard. Outside they have about 30sqft/bird. They also get to spend some time in their tractor and free ranging. They seem pretty happy.
In the end, seeing how they act with different amounts of space (and because I do not free range them, although they have large runs which are roofed and windbreak-ed for wintertime), most of mine get 15 sq ft per chicken PLUS ample run, and I just refuse to give them less than 10 sq ft apiece even in my most 'crowded' pen.
They could certainly live with less.
But they act different. Less relaxed, less individual.
(Also, remember that up here there are many days during wintertime when (despite roof and plastic and hay and all that) they just do not want to spend much time outdoors in their run. For which, having a good amount of indoor space REALLY helps keep 'em civilized with each other.)
My coop is 24 square feet, and the run is 64 square feet.
I have 4 chickens, and I wish that I had more outdoor space for them.
The run is packed down to dirt now and I'd like to be able to move them from various grassy areas so they can eat the greens, bugs, etc. I'll be changing it so that there are 3 areas of 64 square feet that I can rotate them around in.
Most farmers give them 2 sq ft per bird inside the henhouse. But they free range all day. As pets in a backyard, they do need more space because they are confined more.
Well, my 3 girls and their rooster live in a 12 x 12 coop all year round because I have lost two groups of girls and rooster to weasels out here on the high prairie. I had 9 hens and a rooster killed in SEP 07 and then 11 hens and 1 rooster killed this past "Good Friday"...(nothing good about it)
My friends like cleavers and my shelter belt (tree line) is full of them this time of year. I have a fenced in yard for them but can't let them come and go freely because of the varmints that just kill them and leave them without eating them. Hard life out on the prairie if you are a chicken and your human has to go to work...
My parents just fenced in as big of an area as they could... But we only had a max of 10-12 hens, and 6 ducks. If you are talking of dozens or hundreds of birds, then you could start doing calculations. Or maybe if you live in the city and only have a tiny area, then it might be helpful to do the math. I've never seen it to be an exact science, and a lot of it depends on where you live. We had to make our run smaller because of predators. But we don't need a large hen house, because the winters are relatively mild here, and the hens always go outside. Sorry I have no set in stone answer.
like the others have said, it depends heavily on their outside space and how long they will have to stay in the coop in winter. i'd probably go for at least 3 sqft/bird if they won't have to be inside for stretches of time and maybe 5-6 if they will be in longer.
my coop is built on an old hay wagon, so we were completely locked in on floor space. i added some wide shelves to add a bit more room to roam. so my birds have the whole floor space plus a 3ft X 8ft shelf above the nest boxes to wander around on. seems that they prefer the shelf for just hanging out rather than the floor. so, if you are limited on space, that's an option for giving them some more room.