Bigger = better.
Chickens poop. A lot. This produces a lot of ammonia, which can seriously affect your girls' health. And if you're anything like me, you don't clean the coop when there's more than a foot of snow or four inches of mud. (If I can't push the wheelbarrow, it ain't getting done.)
You should be worried about moisture, not cold. Chickens can take the cold. I've had it down to 30 below, and the rooster lost all of the tips off of his comb. That was the only thing that happened. In eight years, I've had one hen die in winter. She choked on a piece of corn (freak accident, but that's what I found in her throat when I wondered why a perfectly healthy hen had died an hour after I fed them.)
Chickens, especially new layers, occasionally like to beat each other up for protein (i. e., cannibalise.) You can prevent this by having a bigger coop.
EDT: also, they're mostly confined to the coop during winter. You want the timid ones to have space to get away from the bored, cranky, evil ones.
Just make sure you don't have drafts and do have ventilation. They should be fine.