Thomas Baekdal has a great report on browser sizes and, more importantly, the size of the browser windows. Complete with a whole bunch of easy to read diagrams and all!
Optimizing your site for the right screen size is important, as every designer knows, but you also have to make the site accessible to others. 1024×768 pixels is the most commonly used resolution, and since the web is overall designed for this nowadays I’€™m not surprised that 80% surf with their browsers maximized. But 1280×768 sees 70% maximizing their windows, which sure will make quite a few websites look like crap.
Check out the extensive and very easy to read report by Thomas. It’€™s a must read!