Google Browser Size is the answer to these questions, it is a free and simple tool that provides relevant information about your site based on data from millions of visitors to google.com. This displays your homepage with information that that lets you understand how everyone sees your website. 

You simply need to enter your website address and your site will appear with an overlay displaying screen size percentage. It lets you see how much of your website is instantly visible to your visitors depending on the size of their monitors and your resolution settings.  

"Special code collects data on the height and width of the browser for a sample of users. For a given point in the browser, the tool will tell you what percentage of users can see it. For example, if an important button is in the 80% region it means that 20% of users have to scroll in order to see it" explains Bruno Bowden in Google's blog.

A very useful tool which can prove out to be quiet handy to determine the placement of action buttons on your homepage. And if you are a web designer, then this tool can help you in redesigning your site and improving it by minimizing scrolling and ensuring that all important parts are easily and always visible to your visitors. 

Give Browser Size tool a try here, and do let us know if you found it to be as useful as we did.