I just tried on my Spyder GS suit which arrived today. I'm 5'7" and 180ish, so by weight I might have been an "L," but I thought height was more important so I got "M"
I tried it on... length is great, but it is hella-tight. (i.e. getting my arms out was a bit of a chore)
I did some body-weight squats and such, and it feels fine... the compression actually feels like it would be good.
But I'm worried that it might tear around the crotch or something... are the suits designed to be tight, stretch, and NOT tear while the movements of skiing stretch it out?