I like to have a minimum of 4 square feet per bird inside (coop) and 10 square feet outside (run) - some folks go as little as 2-3 square feet per bird in the coop. A lot depends on the type (size) of birds and your climate, which dictates how much time the birds will have to spend inside. The more time the birds will have to spend inside the coop, the more room they need to have in there - vs a mild climate where they truly only use the coop as a resting/sleeping place at night. The other reason to build with more room than you think you need is chicken math - better to have the space available for the next chicken, or two or twelve that you just "have" to have from day one than having to build again down the road when you want to add to your flock (chickens are like potato chips, you can't have "just one" - or "just one more")
