Chickens are tough little things.
I'm in Nebraska, we get pretty darn cold out here - I was super worried about my chickens the first winter, I kept thinking i'd open up one morning and find them all frozen to the roost!
Never happened. (our lowest temp last year was close to -40. We did hit -40 with windchill)
I don't heat the coop with anything either - I did try a ceramic heater (for a reptile) but it wasn't putting out much heat at all, it was a waste of time really.
My coop is about 20X7 with another little coop attached which is 7X7 right now we have 14 chickens, 6 ducks and 6 guineas in that space.
My coop isn't perfect either, I have a leak on one side, found snow in the coop last year. One side is insulated, two sides have another coop or the horse barn attached and the front or south side is just plain wood, with two big windows, one window broke our first year - we put up wire, and leave it totally open all summer, we board it up over winter. Seems to work for us.