The rule of thumb on here is 4 sq ft per chicken in the coop and 10 sq ft per chicken in the run. If you get a lot of snow and cold you will need a larger coop for days they will want to stay in all day. In my climate what they need is protection from heat, so my coop could be small and 3 sided and continuous with the run. I actually have about 7 sq ft per chicken with some hardware cloth walls, because they spend the hottest days in the coop for the shade and breeze.
Bigger is always better with chickens. They are cannibalistic dinosaurs and need space to prevent problems.