No such thing as too big. Well except maybe according to your zoning bylaws
It's not real wise to depend on crowding 'em in like sardines to "keep them warm" during the winter. They won't generate that much extra warmth and the only way you'll contain it in such a small space is to have radically-insufficient ventilation, which will tend to produce frostbite at not-that-cold temperatures so it kind of defeats your purpose.
Honestly I think a lot of the reason there are so many people who DO use the "cram 'em in densely and then shut the ventilation" approach, up North here, is simply that they DO get enough frostbite that way that they figure "wow, if they're getting frostbit that much with them closed up tight in a small coop for warmth, just IMAGINE how much worse it'd be with a larger coop and more ventilation!" Which i guess may seem reasonable on the face of it, but happens to be just about 180 degrees opposite to the way things really work
Mine often don't want to go out in the winter, despite roofed and windblocked runs (not that our winters are so Arctic or anything, I think I just have weenie chickens
) and I would SERIOUSLY not want them to have less than 10-15 sq ft per chicken (indoors), and the times when I've been able to give them even more space (thru lower chicken population in the pen) they've sure enjoyed it.
JME, good luck, have fun,
Pat
