Although this is a Team Fit jacket by Volcom, you don't really need to go down a size unless you like your jackets a bit fitted. It is baggier but not huge. I'm 5'10" 140 pounds and wear the size Large. It is a shell but completely waterproof/windproof. A base layer and fleece are good for most days. The length is pretty good, even for someone my height and the arms are long which is super hard to find. There's nothing more annoying then short sleeves on your jacket and you don't realize it until you bend down to do up your bindings the first day you wear it!
I bought these because when I snowboard my regular earphones always come out of place and make my ears hurt. These are more comfortable, stay in place way better, with better sound but if you lose the rubber cushion that sits around the earpieces they are pretty uncomfortable to wear. Actually impossible. They should include extras in the package since they are obviously easy to lose. I think I might try the iCon or Lowrider instead.
Finally! A jacket that doesn't fit all girly! I really like this one because it has a looser fit and you can cinch it up at the bottom to your liking. Also the powder skirt has a bit more room so you can pull it down well below the top of your pants, and then snap it to your pants. It's awesome if you wear your gear a bit baggier (as it should be :)
Also I am a freerider, and this jacket rocks for mild to about -15 celsius (sorry I'm Canadian). The weatherproofing is pretty solid too unless its raining. And as a bonus I got tons of compliments on this jacket from people on the hill.