I wear a 10 in Nike, (27 mondo ski boot), I got these in 9 and they fit perfect. It was snug at first but the toasty removable felt liner breaks in and after it's just right, any smaller would be too small though in the length. It is very wide.
No arch support whatsoever, if you want to put an insole in, you might want to consider sizing up.
I wouldn't wear them to hike long distances since they are sloppy around the ankle and heel, and very bulky.
Warm (hot even), comfy, totally waterproof, and good for walking / standing around in deep snow.