I'm 5'10'' and 144 lbs and I normally wear a 6 or an 8 in pants. I ordered these pants in a medium and they were way too short. The material is very stiff but shiny like a baby diaper.
Furthermore, the cut of the pants is really strange- the hips stuck out like old fashioned horse-jockey jodhpurs. I consider myself to have an 'athletic' (not rail thin) body type and I didn't fill out the hips at all. In my opinion, they just looked goofy like I should have stuffed them with tissue paper.