A question that I often got from my students: how wide should our interface design be? The correct answer is: it depends. I know, not much help, but let me explain.

Most importantly, we do not design website for ourselves. But not even for our clients. We design websites for our clients' target market. And when it comes to the width of the web pages, it really depends on what monitor resolution the target market is using. Once we determined that, we have to consider, that the browser window will have certain elements, such as scroll bar, tool bars, title bar, etc. which we have to deduct from the available monitor dimensions. Therefore, the interface that we design, must fit within the area that is left.

There isn't an exact formula, unfortunately, and that is because the platform (operating system) will also influence the equation.

Here is an article that gives you some guidelines, though. Enjoy.