I am 5'9 175lbs and maybe a 31 waist, what size should i buy?
Size is not directly correlative with height. Instead, you need to measure your torso length. Please find the instructions to measure that here:
My guess is that you'll fall into the Medium range, but you might have a short or a long torso for your size so definitely measure!
I second Kate on that. Measure yourself, or go to a store and have yourself measured with one of their "pack fitters" once you know your size you can order here.