“Browser Size,” is a new 20% time tool from Google, that visualizes roughly how much of a web page typical users will actually see without scrolling. Special code collects data on the height and width of browser for a sample of visitors to google.com. If you're a web designer, you can use Browser Size to redesign your page to minimize scrolling and make sure that important parts of the page're always prominent to your audience. For e.g., if an important button is in 80% region it means that 20% of users've to scroll in order to see it.
To try it, visit Browser Size, enter URL of a page you'd like to examine. The size overlay you see is using latest data from visitors to google.com, so this should give you a pretty good indication of what parts of your UI are generally visible and what aren't.
More info: Google Code blog