I wear a size 10-1/2 men's shoe. My feet are average width. I ordered the Large.

This is only an initial impression review based on fit and feel, as I have yet to use them in extreme cold temps.

These socks are very thick. Thickest wool socks I have seen. I am confident they will be very warm. However, if you are needing a sock to fit inside your hiking boots, you might want to look elsewhere, unless your boots are too big. My winter hiking boots, which fit perfectly with a single pair of merino wool socks, are WAY TOO TIGHT with these on. I tried this for testing purposes only and I am not deducting any points for this because I assumed they would be very thick socks, and therefore did not buy them for this purpose.

These are better suited for the less active winter activities in which one might be wearing a large pack boot, such as sledding, ice fishing, snowmobiling, etc. But for the hiker, stick with thinner merino wool socks.

These socks are very loose, even on my average width feet. They are the correct length but they are much looser than any other sock I currently own. Based on my experience with loose fitting wool socks, this is likely to be a problem inside a pack boot - falling or getting bunched up. I don't recommend these unless you plan on wearing a thinner pair of socks under these, or unless you have very wide or thick feet.

This is the main reason for me docking a star. Although I didn't buy these for use in a boot, I still find them to be looser than I prefer while lounging around inside a home, or tent, etc. They will serve their purpose well for me however - a warm sock for winter camping (sleeping inside a sleeping bag when temps are below freezing).