We all have our own experiences, goals, and personal preferences, many of which are different. We can often give different suggestions because of that. So one challenge is for you to try to figure out if a suggestion actually applies to your situation. And which match your goals and methods. So to start with you need to know your goals and how you expect to manage them.
I'll offer you some reading materials. I think Pat's articles should be required reading for anyone designing a coop. The best way to fix many problems is in the design phase. I may not agree with everything Pat says but it's her opinion and she covers some good stuff.
Pat’s Cold Coop (winter design) page:
https://www.backyardchickens.com/articles/winter-coop-temperatures.47763/
Pat’s Big Ol' Ventilation Page
https://www.backyardchickens.com/ar...-there-and-cut-more-holes-in-your-coop.47774/
Pat’s Big Ol' Mud Page (fixing muddy runs):
https://www.backyardchickens.com/articles/how-to-fix-a-muddy-run-chicken-coop.47807/
Next I suggest you follow the link in my signature below about space. I personally don't put any faith in magic numbers like square feet per bird, if you follow that link you'll see why. We are all unique so what works for one does not necessarily work for another.
I'll include this article since you are so worried about cold. There is some good stuff in it but my main purpose is so you can see that you may not be as cold as you think.
Alaskan’s Article
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
Now, some of my opinions and personal preferences. You are as important as the chickens, if not more so. Plan for your comfort and convenience. The chickens will benefit from that because if it is a pain to take care of them you won't do it that well. The chickens will adapt so take care of yourself.
For anything over 8 chickens I would want a walk-in coop. You need to be able to access every part of the inside for many different reasons. Once you get over a certain size that is not easy.
I also like a dirt floor. If you have a solid floor you may be giving Mama Mouse a convenient and safe place to raise a family near a good food supply, your chicken feed. In a cold snap the dirt floor acts as a thermal mass, can warm the coop up a bit. In the heat of summer it will probably be cooler than your heat waves. A dirt floor doesn't rot or decay.
A wet coop or run is an unhealthy coop or run. When I built my coop I added a couple of inches of clay dirt to raise it above the outside level to keep the rainwater run-off from coming in. I built a gentle berm and swale on the uphill side to divert water away. My coop stays very dry.
When I look for a location to put a coop and run I first look at drainage. If water drains to an area or that area stays wet you will probably have issues. If water drains away from it you will probably be OK. Pay close attention to Pat's muddy coop and run article. A dry location is my top priority.
If you built a little bitty coop with no access you have no choice but to build a nest that you can access from the outside. If you build a walk-in coop you have an option. There are plenty of legitimate reasons you may want to have outside access but my preference is to go inside to collect eggs. I go inside to feed and water anyway. I've found a dead chicken, a couple of snakes in nests, and even a young possum in the coop by going inside that I might have missed if I collected eggs from the outside. Well, I'd have noticed the snakes but maybe by feel instead of eyesight. Going inside lets me check up on the flock and the coop.
You want the roosts higher than any place you don't want them sleeping. Chickens tend to want to sleep at the highest spot available. The ones highest in the pecking order get to sleep wherever they wish so be generous with roosting space.
The way I determine vertical spacing in the coop is to first determine the height of the coop floor. Include thickness of any bedding. Then position the nests. Some people like the nests on the coop floor, others put them high enough they don't have to bend over to gather eggs. Think of your comfort and convenience. The chickens can adapt.
The roosts need to be higher than the nests to try to keep them form sleeping in your nests. I want the roosts clearly higher than anything else but I also want them as low as reasonable. The higher the roosts the more clear space they need to fly down from the roosts without banging into walls, nests, feeders, or waterers. Also collecting them off of the roost at night is about the easiest ways to catch them for inspection, treatment, or anything else. So have the roost at a convenient height for you.
I personally like "droppings boards" under the roosts. They poop a lot at night while on the roosts. Since they are not walking around in the dark that poop can build up. Some way to gather that poop makes keeping it clean easier. There are all kinds of ways to do that. Some people hang hammocks under the roosts. I've used plastic bins setting on the floor. Some people build trays and keep different stuff in those trays. I also use a flat piece of plywood and scrape it clean.
I'll stop here, that is a lot of reading. Good luck and stay flexible. Nothing ever works out exactly as planned but as long as you take care of the basics it's really not that hard.