It may seem like a silly question – but it isn’t. Just because people have larger screens it doesn’t mean that their windows are open full screen – some users don’t even know how to go full screen. Some how much of your website page is actually seen without scrolling. A question that can have quite an impact on how you design a page and where you put a vital massages. Many designers concentrate on above the horizontal fold i.e. where you have to start scrolling when full screen (on my screen that is at about 650 pixels) but you should really be working from the left as well – think about a vertical fold. In addition it is also important to allow your webpages some sort of design flexibility especially on width. Modern design tend to lock a website to a specific width (favourite is 960 pixels) but some flexibility in this should be considered.
Google Labs have been running a very interesting experiment mapping screen sizes to percentage visibility see these links.
The browsersize link could turn out to be invaluable to designers and usability experts.