If that's case, enjoy your bar ends.

To answer the original question, I don't know my shoulder size, but it's probably about the same as my road bar width, which is 42cm. I use a 71cm bar and am a firm believer that road ≠ mtn when it comes to setup needs. You'd be better off erring on the long side than the short side of bar length, and you may need a shorter stem as well. BUT, it really depends on the kind of riding you want to do.