For example, in the screenshot, the 90% contour means that 90% of people visiting our website have their browser window open to at least this size or larger.
The sizes represented by the contours are client area sizes, not browser window sizes.
This means they represent the size of the browser without the title bar, toolbars, status bars, etc, and thus give a true representation of how much content can be seen by a particular segment of Web-users.
You can view any website with the same overlaid visualisation by entering the URL of the site you want to check.
Why you should take a look at Browser Size?
Having access to this sort of data is no doubt valuable.
But what I find really special about this tool, is having the ability to quickly check how visible the key elements of a user interface are to a wider audience then your own analytics data could ever provide.
That said, it’s important to note that Browser Size works best on web pages with a fixed layout aligned to the left. For other layouts the results can be misleading.
Google state they are actively looking to improve this tool so expect these issues to be resolved over time.