My chicken shed is 12 x 8 divided into 2 different areas that can be open or closed from each other depending on circumstances.
I found that it was too small for 11 birds last winter when they are wanting to be inside more. I also have about 20x20 kennel run on the outside which is nice, but in winter some of the more aggressive of the flock would block some of the "lower" members outside when it was much too cold. There were a couple days that I didn't open the door because it was brutal cold and I didn't want any getting stuck out there. (Though I did have a dog house out in the run too that they could get out of the wind in if needed.)
When I had to keep the door shut it was WAY TOO MANY BIRDS for the small space.
I remember hearing someone saying it this way:
Imagine your smallest bathroom. Now imagine your whole family crowding into it. Then invite over a couple of grandparents and other extended relatives and have them all come in too. Now imagine that you have to eat in there, drink in there, sleep in there, potty in there. And you have to do that from November all the way through the end of March. What do you think might happen to your relationships/patience with each other?