...To set the maximum width for the content, put html
{ max-width:1280px; margin-left:auto; margin-right:auto }
in your monobook.css (adapt "1280px" to the width you like
to keep it working normally on smaller screens)...
...But aren't there inferior popular browsers that fail to support the 'max-width' attribute? ... It would be better to use a relative measurement such as em instead of px, to allow for it to rescale if the font size is adjusted by user preference. Otherwise, you can have a situation where somebody sets their normal text to a large size (due to poor vision or an extremely high resolution monitor where pixels are very small) and still ends up with it crammed into a fixed-pixel-width canvas...
I just adjust my browser window to ~40 characters across for easy reading, or I print it out and read off line. I've also browsed through Adobe Acrobat (writer) and read on screen or on paper. There are options, and maybe we need a page to remind people what are their viewing adjustment options. At Wikipedia, [[help:____]] there are only 2 articles with the word "display" or "font", 1 with "view" or "size", and NONE with the words "adjust", "screen", "resolution", or "window". What name should we give an article that tells people how to adjust their display screen resolution and font and window size for optimum reading?
I think % is relative regardless; em is un referenced to screen physical dimensions but should correspond to exact measurements on print output devices; and px depend on device-controlled physical dimensions that vary from device to device. Why bother? Everyone's display and system rendering is different!
Regardless, 508-compliance requires that information on screen not be dependant on color (hence my preference for traditional underlines versus red or blue for links), be responsive to user adjustments for contrast and size, and present themselves to screen readers with equivalent information for non-sighted users. Have a look at http://www.section508.gov/ and report back in a few days. Nice "Site Layout Controls" in the upper right corner, eh? ;-)