Google Browser Size is a visualization of browser window sizes for web sites. Enter a URL into the app, and the page is displayed behind an overlay showing the percentage of browser users who are able to view the visible portion, based on their browser's available viewport size, and their display's screen resolution. For example, the "90%" contour means that 90% of people visiting the site have their browser window open to at least this size or larger.

http://browsersize.googlelabs.com/