hmmm, why don't you just count all the pixels? I would say either copy and past it into photoshop, or go into the CSS file and see what the width is set too.
Lol, This of course will depend on the screen resolution in most cases. If the site is fixed width, you could of course take a screenshot and dump that into an image editing application, then use some tool to measure the width and height. If it is just the one site you need the dimensions of, post a link here. I am sure somebody will do it for you.
As said the qustion is vague Could be rsolution size If you mean the content, then they may have defined the width it in the css under something like content or wrapper. If you just want to measure something then use the measureit plugin for firefox http://www.kevinfreitas.net/extensions/measureit/ If you have web developer toolbar installed then the ruler is under miscellaneous =>ruler I think