That blog is pretty good but you might follow the link in my signature for some more thoughts on how much room they need. There are some things in my article that you might want to take into consideration. For example, I don’t consider how much space is in the coop by itself as important as how much space in total is available when they need it. That can be in the coop, coop and run combined, or they can free range and sleep in trees. I look at it more as a system than in individual parts. How you manage them has a tremendous effect on how much space you need.
I don’t know where you are located, but they probably need more room in winter than in summer if weather will keep them confined. The more crowded they are the more behavioral problems you can have. That example of the elevator and diner in that blog is very good.
Their permanent down coat will keep them warm as long as they have a minimum of protection from direct wind hitting them. In tight quarters in winter they need really good ventilation as well as space. They put off a lot of moisture, through their breathing and from their poop. If that moisture cannot escape, that can lead to frostbite any time the temperatures drop below freezing. With no moisture build-up and with a direct wind not hitting them they should do fine well below zero Fahrenheit.
Another potential problem with poor ventilation is that ammonia can build up from their poop. Ammonia is hard on their respiratory system. But ammonia is lighter than air. With openings above their heads that ammonia will escape instead of building up. That’s how you handle ventilation for moisture too. Have openings above their heads so any breezes that come through those openings are above their heads when they are on the roosts.
In the summer it doesn’t matter where those openings are as long as you have plenty. Heat kills a lot more chickens than cold. Heat is your real enemy. Don’t worry about keeping them warm. Their down coat will take care of that. Worry about them overheating.
