The service can be used for any web page, but the data is obtained from the visitors of google.com. As you can see from the screenshot, Google's top result can be viewed by more than 99% of the visitors if no ad is displayed above the results.

Google Browser Size is one of the many Google tools that help you optimize web sites:
* Google Website Optimizer - testing and optimization tool
* Google Analytics - web analytics
* Google Webmaster Tools - site performance, crawl errors, top search queries
* Page Speed - an open-source Firebug add-on that helps you evaluate the performance of a web page.
* Speed Tracer - a Chrome extension that helps you fix performance problems in your web applications.
* Closure Compiler - a tool for making JavaScript download and run faster.
* "Let's make the web faster" tutorials.
{ Thanks, Kevin. }