I purchased the large and they fit perfectly.  I'm 5'9" and 150 pounds.  Then I washed them and threw them in the dryer.  They're more like a medium now.  But I figure any pair of long johns I can't dry and put on toasty from the dryer isn't worth having so it was a good test run.  I'll donate them to Goodwill and buy another pair of the Duofold mid weight which were not only more comfortable, they are the same size after being washed and dried.