I worried about this as well after reading articles about coop sizes. My coop is a small house that was used for the previous owners mother to live in. I have 5 chickens that are housed in the "living room" and 6 ducks in the "kitchen". There is a partial wall and door between them but open at top for airflow. It was already built like that.
The bedroom area is closed off until spring when we plan to add more to the flock. The ceilings are probably 8 feet high. Last winter it was used for storage and when I would go out it would be absolutely freezing. I'm in Montana. Now that it's occupied it stays a comfy temperature and everybody is happy to have plenty of room for activities.
