If you're close to fitting into a regular size, go to the large -- the regular only fits 5' 5" not 5' 6" as this site states. It says so right on the Mt. Hardwear bag tag, so go figure. Anyhow, if you're a little over 5' 6" you will find this bag to snug and your feet will poke out in the toe box. Otherwise, great lightweight backpacking bag.

First off, I typically love Montrail shoes so it was my main choice when looking for a cross-over shoe for trail running and lightweight backpacking (3 days in the Sierras with about 30lbs on the back, plus non back days peak bagging, fly fishing etc.) When the shoes arrived, I wore them around the house for a few hours and found that the plastic buckles at the ankle dig into your skin. I was not overly tightening the shoes either. Additionally the toe box felt a little less roomy than normal for Montrails. The company's size 8 typically fits my feet fine (slightly wide toe box but narrow elsewhere). The shoes would not accept my thicker Smartwools or new Keen trail running socks without feeling too tight! A size 8.5 might. So I left the shoes at home to be returned and got by with my regular trail runners and only a couple of blisters. I do like the cross between backpacking/trail running in a shoe but this model needs more work. In the meantime I'm going to get the Asolo Styngers and stick to more traditional hikers (although lightweight for their beefiness) and cart a pair of trail runners with me for scrambling.