Nicholas,

I'm about your same size (5'8" 160-5lbs) A medium will probably fit, but I prefer a large for the extra layering options. This jacket is a "slim fit" but that doesn't mean much for folks built like you and me. Look at where the shoulder seam fits on whichever size you decide to get. On a good fit, the shoulder seam will be just beyond the edge of your shoulder bone giving you plenty of mobility. If your shoulder is at all restricted, go a size up. And if you hate it, say so! BC.com is really good about returns and exchanges on sizing issues.

Happy trails!

Nicholas,
I'd say Medium, or even a small for you. With the fit of the Down Shirt you don't want to go to big or it drapes funny. I'm 6' 170 and a medium is right on the money for me.