I bought two pairs of these in size S, I wear women's shoe size 5.5--they fit perfect.  The socks are a nice thickness to provide enough cushioning during long hikes.  The warm level is fantastic.  I have been hiking in snow with normal snow boots (i.e. rated to 15F, not for -5F for example) and have been able to hike for 3 hours in deep snow with this single sock without much trouble in 25F.