The "standard" answer is 10 sq ft per bird, which is supposedly the minimum needed to prevent behavioral problems (serious pecking, feather picking, cannibalism.) I think most people would answer, as big as you can make it. My 9 chickens have a large yard, about 5,000 sq ft. They keep the grass eaten in it, and if the gate is opened, immediately leave to forage elsewhere.
when referring to square footage...you really mean VOLUME, don't you? In other words, my indoor coop area is 4'l x 8'w x 6'h making it 192cubic feet. Is that correct? So with 6 birds...calculating 10 feet per bird, I have enough room.
It's the square footage of the usable floor, that they can walk around in. The rough dimensions of length and width multiplied, then subtract unusable space that's taken up by feeders, water fonts and nest boxes. This is the traditional way to do it.
For large coops and runs, I don't worry much about subtracting for objects. In the small coops and runs, I think it's very necessary.
Volume isn't the issue, Out portable hoop runs are only 3 feet tall they ate 4X6 (24 square feet) They were made to fit on top of our raised beds so the chickens can weed, aerate and fertilize between crops. I usually put two but sometimes three hens one.
We free range (1/2 acre) 80% of the time but we do use our run when its raining (often) or when everyone will be away from home for longer than a few hours.(seldom). The run is 7x12 feet and we keep 9 hens at most. However if they were going to be stuck in in the run 24/7 is really only big enough to comfortably accommodate six standard size. chickens.