Leave it to Google to come up with a super-easy way to compare your website to the average users’ screen size:
Google Browser Size is a visualization of browser window sizes for people who visit Google. For example, the “90%” contour means that 90% of people visiting Google have their browser window open to at least this size or larger.
This is useful for ensuring that important parts of a page’s user interface are visible by a wide audience. … To view your own Web site with this same visualization overlaid on it, simply type its URL into the “Enter URL here” textbox at the top of the window and click Go.