Bigger is better if you can. I think the majority of flock behavioral problems (like picking and causing bleeding) are from overcrowding and boredom.
Then techincially I was fine before BUT even better now. Brooder is at least 10 by 10 and I thing the little dux coupe / aka annex is about 14 by 12. I have 10 in the brooder and 15 in the other. And mister angel wing has made the decision on at least one that will be culled.