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.
Optimising Digital Marketing Campaigns with Search, Social and Analytics
At SES London (9-11 Feb) you'll get an overview of the latest tools, tips, and tactics in Paid, Owned, Earned, Integrated Media and Business Intelligence to streamline your marketing campaigns in 2015. Register by 31 October to take advantage of Early Bird Rates.