Actually... its with simple mathematics. I have been running forums for years, changing widths, trying to find that sweet spot. I eventually settled on 990px just a few months ago with the switch to XenForo... 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.
This is how I do it on my forum:
Code:
#content
{
padding-left: 10px;
padding-right: 10px;
}
.pageWidth { min-width: 990px; max-width: 1200px; width: auto; margin: 0 auto; }
As you can see, it sets the width of the page as 990px, and is expandable up to 1200px. It also centers the page by margins. It also pads the content box 10px on each side in order to retain the margins. You can see how this works on
http://www.8wayrun.com/
Also, 990px is pretty much what you would need to use XenMedio's 640x360 media player with stock skins.