i am in northern michigan and we can get very cold up here. i don't think it matters how large/small it is, your chickens won't heat it with their body heat. remember, chickens wear down coats. as long as they have plenty to eat, and are DRY they can actually withstand a lot of cold, especially if you put a heat lamp out for them. it isn't the cold and dry, it is the cold and wet that causes problems. my girls went thru the winter in a stock trailer. i had some lights on a timer to give 10 hours a light in dec, jan and feb. in the middle of feb the lights were turned off. from 4 chicken i got 3 eggs a day, all winter. the front half of the 16 foot long trailer had plexiglass over the openings, and that's where their perches, nest boxes and feeder was. they were out of drafts, but the trailer had a lot of venilation. the back half and the back end's openings were not covered and the waterer was in the back half. i snow-blowed clear up to the trailer, and shoveled the last foot or two to get in. my girls were able to free range, uh, snow range all winter. they explored the lean to and the tractor shed. i was pleasantly surprised because i put in one bale of straw last october and it is all still dry except for the one wheelbarrowful i took out just today. there is no smell whatsoever. i fluffed the straw about once a week and tossed leftovers, scraps and scratch to encourage them to keep it churned up. i would say not to worry about keeping it warm. if it is closed in enough for you to keep it warm, it is closed in enough to stink. i think they do better with more room. moving around gets muscles warm and moving. the one thing i am going to do next year is to up heat lamps over my night roost, but i think this is more for my peace of mind. also be sure to use wood for perches, nothing metal, that can sap warmth from their feet. if your roost is big enough for them to hunker down on their feet (i have the 4 inch side of a 2x4) i think you'll be surprised at how well they do. i think our birds are tougher than we give them credit for.