I don't think a coop can ever have too much ventilation, even when it's super cold. I have operable windows on three sides - and on the north side where there is no window I have a cutout covered with hardware cloth. It's 8 inches wide and 2 feet high. That's the only one that gets closed off after winter settles in because it's the side most of our nasty Wyoming winds and snow come in. No matter how cold it is, windows on the lee side of the coop are open, and closed only when the winds and snow are coming from that direction.
I also have operable vents high up on the west and north sides of the coop. They are never closed. I have a vent down low on the west side of the coop, about 8 inches from the floor. The pop door on the east side is open 24/7 as it opens into their secure run. A real find was a mobile home side venting mobile home fan. It's above the people door on the coop. I found it on a site that sells parts for old mobile homes, and it's perfect because the walls on those older mobiles are thinner than today's construction standards. So the fan fits well. We have it wired so that it can either be passive (just the cover open) or active ventilation (the cover open and the fan running) and I use that in the summer.
Ken laughs at me and says the coop is more openings than solid walls, and he says he could have saved a ton of money and building time if he'd know ahead of time how much siding he'd have to cut away. But my birds love it, the coop stays nice and dry, and I have some flexibility as far as protecting them from wind yet providing ventilation.