You can either have a liquid layout so it resizes to the size of the browser or a fixed width size. If you're going fixed width go for a size thats generally supported by the majority of internet users, for instance go 1000px wide for a minimum screen resolution of 1024px * 768px (the missing 24px allows space for the scrollbar and browser border). Both methods would be controlled in your css file.

It used to be a case of setting up fixed width sites for 800px *600px users, but web stats show us the majority of users use 1024px * 768px or higher. Check them out here: http://www.w3schools.com/browsers/browsers_display.asp