you talkin' Ether here? if so, i'd say go with the large. i'm much shorter than you and wear a 40 extra-short suit. a medium Wisp is a slightly snug fit, getting it on & off just over a base layer. fits fine, once on. could also wear it over a thin wool mid layer, but nothing thicker/more insulating. sleeve and torso length is fine for me. my gut feeling is that it will be too short for you. my wife is a couple of inches taller than me and slim and wears a medium Ether.
if i see my son tomorrow, i'll have him try it on - he's just a tad over your ht, but built slimmer. at least, it will be a reasonable check on sleeve length. i believe the sleeves and torso length will be way too short on him, but i'm just guessing here based upon how it fits me.