Let's say that you want to have 10 chickens. You've done your research and found that 4 square feet of floor space is about minimum for a comfortable, adult bird. So, 40 square feet is needed for your 10 chickens.
The rule of thumb is 3 cubic feet per pound of live chickens, permanent indoor confinement (Virginia Polytechnic). This can be thought of as an "industrial model" but, hopefully, your birds will have outdoor time most every day.
10 chickens weighing only 5 pounds each or 50 pounds of birds would "require" 150 cubic feet.
If your 40 square foot coop is only 3 foot high, you have only 120 cubic feet. If your birds are larger than Leghorns or bantams, you don't have anywhere near even the "industrial model" for room.
I have a coop with 2 rooms, 1 is fully open to the outdoors. The other room is fully closed and insulated for the Winter. The hens will stay in that closed room every night and many of the Winter days.
It has only about 150 cubic feet of air space but I've never had more than 30 pounds (
) of chickens in there. Their roost is 18" above the floor but there's open air above them.
Steve
edited to provide a reference