Some websites state that they are designed for a given resolution or even that they require a fixed resolution. I recommend designing a website in such a way that it has either a width of circa 600 pixels at maximum or, even better, to use liquid design. Even though many website designers argue that most people have 17" monitors and use a resolution of at least 1280 x 1024 pixels. However this fact doesn't state anything about the actual size of the browser window.
The advantage of the 600 pixels limit or the recommended liquid design is the possibility to view two browser windows on a 17" inch monitor side by side.