Browser size that is! Visitors (potential customers) to your site sport an array of different sized monitors. And their browsers may not always be full-screen, especially when they have toolbars for bookmarks, navigation, search boxes (e.g. Google, Yahoo, etc.), virus alerts, etc.
Google has developed a free tool (Browser Size) to help you understand how to take browser sizes into account when designing your website and/or landing pages. Per Google:
“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. On the example page that you see when you first visit this site, there is a “donate now” button which falls within the 80% contour, meaning that 20% of users cannot see this button when they first visit the page. 20% is a significant number; knowing this fact would encourage the designer to move the button much higher in the page so it can be seen without scrolling.”
The Browser Size tool could be useful in determining the best placement of crucial content, call-to-actions, banners, sponsorships, etc.
Read more about how the tool came to be at this Google Blog.