This is the general rule for square footage: 4 square foot per chicken inside the coop, 10 square feet per chicken inside the run. A little bigger is always better. Hope this helps!
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4 sq. ft. per bird minimum so 4' x 6'. I would build it bigger because you will probably get more chickens!
There's some school of thought that you can get by with less sq. ft. if the birds are out of the coop in a run or if they free range most of the day. You don't need so much space if all they do is roost in there. I still like the idea of a large coop because you never know when and how long inclement weather will be. I think you need 1 nest or laying box per 4 pullets/hens. I want 1 per bird in case they all go BROODY at the same time!