I think most people would want a medium (since they don't sell a "large"). I am not a very large build, and I run my medium at the narrowest setting--there is a lot of expansion on it though depending on which connectors you use.
I bought one of the adjustible bike-specific versions which allow for more movement than the MX versions, but I run it in a more restrictive setting (guard extended up) than I expected I would. It works great with my Giro Remedy in that setting. I tried it with my Specialized Diviant (FF XCish helmet) and the rear didn't feel very good. Since I was on a group ride, I didn't spend much time playing with it though--I think I could have made it work better with a little play time.
I bought the white version which was a mistake. I've only used it twice now (purchased end of season last year), and already it looks really dirty. I should have gotten black... That being said, I'm very happy with the brace otherwise. I always used to joke that I had armor on everything except for my neck and my nuts--now at least one of those two areas are protected.
