I wish I could answer that question. Since I've bought L and I'm 6' regular arms I would guess arm length could be a bit short if you will go with M. Good thing about this jacket is that bottom (as well as arm ends) has finish with rubber inside - that keeps bottom and arm ends close to your bottom and wrists but not very tight. I hope that can help. Overall I love this jacket, more than Marmot down jacket I'd just sent back to STP. The worst case scenario you would do the same.