The tool was is called Browser Size and was developed in the famous 20% time, where employees are free to work on their own projects. When you type the URL of a website into the tool, an overlay displays the percentage of average web users that can view a section of the site without scrolling.
These percentages are based on sample visitors to the Google web page. You might be surprised how many web users are using very old-school browsers or old computers/operating systems.
You'll want to check your analytics before making any drastic changes based on what you learn from Browser Size. If your audience is more tech-savvy, for example, then the percentage of your users seeing more of your website will be higher.
