Official Google Blog: Browser Size: a tool to see how others view your website

To help you understand how everyone sees your website, we created a tool called Browser Size in our 20% time. Browser Size is based on a sample of data from visitors to google.com. Special code collects data on the height and width of the browser for a sample of users. For a given point in the browser, the tool will tell you what percentage of users can see it. For example, if an important button is in the 80% region it means that 20% of users have to scroll in order to see it. If you’re a web designer, you can use Browser Size to redesign your page to minimize scrolling and make sure that the important parts of the page are always prominent to your audience. We hope people will use this tool to make their websites better, in turn making the web better for everyone.
(Link: Official Google Blog: Browser Size: a tool to see how others view your website)