You're going to get a hundred different answers to your question. Here's mine:
I'm in the Cities. My coop is 6x8 for 4 hens. It is fully insulated, both walls and ceiling. Four birds CAN NOT heat up a large space sufficiently and, for me, heat was mandatory. Your coop will be smaller, so that will be better for heat, but harder on the birds when they have to stay inside for a week at a time because it's -20. And venting will be more challenging in a small space.
I would recommend running electricity to your coop, or at least have it near an outlet so that you can get a cord to it. You're going to need a heated water dish unless you plan on changing the water out pretty much hourly, and honestly, who is going to do that?
I chose a 250 watt ceramic heat emitter for my heat source. Bought it on
ebay, but they are also available at
Petsmart in the reptile section. It's hard wired over the roost and operated by a switch. But I ended up leaving it on 24/7 this winter. The socket must be ceramic as it gets VERY hot. I blew the dust off it every other day. A ceramic bulb provides radiant heat, which heats up surfaces, not air. Even so, it kept the coop about 10 degrees warmer than outside, which is just enough to take the brutal edge off, but still not that wonderful when it's -20. And we both know how many days we had of that kind of cold this past season. When it got REALLY cold, I added an auxilary infared lamp for overnight. And yes, the threat of fire is very real, so consider your heat source carefully.
My only regret in designing my coop is that I didn't hardwire TWO light fixtures, both on switches so I could easily use the additional light when needed.
Ventilation will be important, so do a search - there's a TON of info on this site.
Feel free to pm me. There are a ton of us BYC folk here in MN, so you'll likely hear from many more. You can come over and I'll show you my coop if you're close by.
good luck and enjoy the process!