I'm a size 11.5 (US) and ordered XL; they're a bit on the big side which I prefer in a slipper.  They look good and are comfortable, though the lining makes them too warm at times.  They did wear out fast.  I wear them for a total of about 6 hours a day, mostly sitting and they are falling apart.  At one month they already had a hole in the bottom and so became useless when it's damp outside.