/tools/css/themes/dreamy/main.css
#wrapper { margin:0 auto; width:700px; background:#ffffff; font-size:1.20em; }
#wrapper { margin:0 auto; width:960px; background:#ffffff; font-size:1.20em; }
#sidebar { float:right; width:180px; background:#efefef; margin-right:10px; }
#sidebar { float:right; width:300px; background:#efefef; margin-right:10px; }
#content { float:left; width:500px; min-height:400px; }
#content { float:left; width:640px; min-height:400px; }
Which theme are you using? And what resolution screen are you using? Bear in mind that not everyone has modern hi-res screen sizes, so don't make it too wide. I tend to work with a maximum width of 980px, so that people with a screen resolution of 1024x768 wont have to scroll left or right. The other option is to use a layout that will stretch to fit the screen, but getting that to work consistently across the ever growing range of screen sizes can be a real challenge.
Jon