I bought the 10 regular. I am a size 8, sometimes 10 and 5'7" tall. I was looking for a pair of heavy duty pants for hiking at high altitude in the cold. I actually picked another pair for that trip but had to keep these too because the fit was so nice. There is room to put long underwear under them which I wanted. I don't need them but will most like use them to ski. I haven't used them yet but Arc Teryx is such a great company.
Hope that helps