Montbell sizing is great for the tall/skinny of us.
I'm a little over 6'2", 180, 41" chest. The Montbell size Large is basically perfect for me. I bought one for a friend's son's graduation and he's skinnier than I am (and maybe 6') and a Large still fits him fine. You often see advice on Montbell sizing that you should "size up" in their down layers and I don't agree. If you're slim to medium their sizing is great.
I tried a Medium Tall First Ascent last month and it was actually too snug in the chest. I'd guess it was 38" or 39'? Arms and torso were plenty long. Oddly the arms were huge in width–enough room for a major body builder! Even if the chest had fit I probably would have sent it back anyway because of all that dead space in the arms.