i think it runs a little big. i bought one last year, and i usually wear a 2X in most things, so i got one. it's a little loose and bulky for my liking. i did try it, on, but it was a closeout and was the last one they had, and the price was unbeatable.
that said, it is well-made and has an interesting waist-cinch feature that will certainly keep the wind out. this part is also removable by unzipping if you don't need it or prefer not to have it.
the pattern i have is the green, and of course that's awesome. i would say be conservative on the sizing, and go for it.
edit - i just looked on their website, and their sizes are clearly listed. i could have gotten by with the XL as listed. so, i guess they are pretty true-to-size after all.