I always seem to suffer from chilled feet when winter backpacking. Maybe it's just the way I am or maybe it's what I lie on at night or maybe a mix of the two. What I've used for years is a three quarter length Thermarest Ultralight with my legs from the knees below overhanging. To the Thermarest I attach a length of cell foam mat which is attached by a strip of Velcro. This is fine for much of the year and the two, when I'm on the move, are rolled (the Thermarest deflated) inside my frameless backpack.
However, I decided that enough was enough. When it's really cold outside, cold feet are a problem, causing sleepless nights and discomfort. I decided that the NeoAir Xlite was the possible solution and the main reason for going for it was its light weight and small packed size. A full length one was essential in order to keep my feet off the cold ground. The lengths for men are Small (119cm), Regular (183cm) and Large (196cm). My length from top to toe is 167cm (5ft 6ins in old money) so none of these was quite right. However, they do a women's one in a single size of 168cm. This has a slightly higher heat rating (3.9 compared to 3.2). Apparently, women need a little more warmth than men. NeoAir refer to this as the R-Value. I decided that I could do with this higher rating as well and the size was spot on. The women's also has an almost indistinguishable pattern on the upper surface. All NeoAirs come in one colour - yellow.
It arrived quickly and I had to try it out, even if it was in the back garden at home. As it happened, there were several inches of snow. Perfect. I cleared some snow away, just enough for my body outline and set up the tarp. Using a tent would have meant more snow clearing. It didn't snow any more during the night but the temperature dropped to -7C. My feet started reasonably warm, I wore thick, warm socks and was in a down sleeping bag rated to -5C. Result! I spent a very comfortable night and overslept.
Bought online from Gaynor Sports of Ambleside for £84.99 in their sale. Very good value.