It's simple mathematics. I have been running forums for years, changing widths, trying to find that sweet spot. I eventually settled on 990px... Allow me to explain why.
In most browsers, the width of the scrollbar is 18-21px. So in a 1024 width monitor (the baseline for websites these days), this immediately means you must throw out 21px, giving you a finite width at most of 1003. No website should EVER be longer than this (or if you do longer than this, allow resizing using max-width, min-width).
If you look at the default XenForo width, it lists 933px (margin: 0px 35px ). This is way too small, that is a full 70px you are completely wasting... thats 7% of your very valuable screen estate. It should definitely be increased. And overtime, I have determined that you can increase the width to 990px, and still retain a semblance of margins. Also, 990px is pretty much what you would need to use XenMedio's 640x360 media player with stock skins.