I was looking for a piece to use as an insulating layer under a shell on really cold ski days in the Tahoe area. Our weather rarely goes below 15 F, so a full-on down or synthetic parka is too much for me. I am 6'4" and 210 pounds, so I originally chose the XXL size. This size usually works with me for most outdoor gear. WOW,the XXL was like a tent! I returned it for an XL and found that the arms were a little shorter, but still reasonable for under a shell. The problem was that the body area was still too full and puckered up under my shell. I think that the Nano Jacket fits a little tighter, but I went with a Mountain Hardware Nitrous Down Jacket, as it fit me perfectly in XL and felt great under the hard shell.