learning more about screen sizes

Looking online to understand how my site will perform on different screen sizes and browsers.
What happens when a user is using a browser and has the window reduced down to a small square or rectangle?
I want to look at making my site have a re-scale area so that if the window is reduced the outside area will disappear leaving the actual content of the site fit flush into the space but reduced down appropriately so that all content is still viewable on one screen.

http://www.tripwiremagazine.com/2012/04/responsive-web-design-tools.html