Presumably you're using tables or divs to hold things in place, if you really want to use all the window space available, allow the body text out of the restrictions you've set for it.
On the other hand, trying to read text that flows all the way across a very wide screen (>1024x768) is something most people find irritating or tiring on the eyes.
Me personally, I don't have any windows taking up fullscreen except the very occasional one for a specific purpose, and I'm running 1280x1024 res. It annoyed the hell out of me recently when going to a website that had some stupid screen size detection code, assumed I ran all my windows fullscreen, and as a result I have to look at a website whose dimensions are all out of kilter.
Fullscreening windows as a rule for any display >1024x768 is a major waste of space. Most websites will have most of the window blank. At a guess, the average browser window size on my machine is ~800x600.