Just had this out on Mt. Washington for the first time. The first two days we were not able to summit because of heavy wind and snow. I was completely comfortable in this jacket, a Smartwool baselayer, and a quarter zip light mid layer in 70 mph winds with gusts up to 105. This thing is bombproof. I started with the Proton AR under it, which came off almost immediately. My point is that this thing really keeps you warm, which makes it a great winter shell. The pit zips also helped dump heat when necessary.
I am a muscular 5 feet 10 inches, 190 lbs with broad shoulders, 32 pant size, 42 - 44 chest. I tried both a medium and a large and went with the medium. It fits the Proton AR or Atom LT under it with no issues at all. I cannot really ever see myself needing any more layering than that with how windproof it is. I can easily fit a puffy over it if I needed to, which I never did. Interestingly, the regular fit medium SV fits almost identical to my large Alpha FL, which is the trim fit. I am a little obsessive when it comes to sizing and was back and forth with what size to keep on this one. After putting it to use I am glad I kept the medium. It fits a nice midlayer under it, but is also not too baggy with just a thin layer either. As always, Backcountry.com has great customer service and has never given me issues with any of my returns or exchanges. I hope that helps!