You can get away with having a smaller coop, if your run compensates for the lack of space;
I tend to disagree with this statement going into winter. You can cheat in the summer, but not in the winter. Even if your coop is open to the run 24/7, the darkness will make them all go to the coop, yesterday, mine were heading to the coop by 5:00 pm. The long winter nights will artificially confine them in the coop, and if you don't have enough space, you will get bad behaviors.
You can cheat in the long days of summer, but not going into winter. In the late fall, you really need to fit your coop with out cheating, or you and your birds will regret it.
Question 2.) I'm building a new tractor coop for 6 Dominique's and 6 light brahmas the run is 8 feet long and 5 feet wide about 7 feet tal mostly covered by a tarp the coop is 5 feet wide about 5 feet tall and 4-5 feet deep is this enough space atleast for winter for these 12 birds during summer I open doors
No, 25 square feet would fit about 6 birds. 12 birds in that tight of space are going to produce a large amount of moisture, and they are going to be too close to the walls and maybe the ceiling. There might be a lot of frostbite from the moisture build up, and more than likely a lot of fighting during the roosting period which will be probably close to 14 hours a day.
Edited by Mrs. K - 11/15/15 at 11:41am