Google has a helpful tool to visualize what different visitors to your site see in their particular choice of browser. From Google’s project About page:
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.
Check it out! http://browsersize.googlelabs.com/