Any web designer should know how to do this... the ability to accurately resize your window to any of the major screen resolutions to test your designs.
Just type any of these commands into your browser URL textbox
Another small trick, is to right click, and choose to add the link to your favorites list. You'll get a warning about the link possibly not being safe, then make sure that the ":" is after the word javascript (it strips it out when I try it for some reason). That way, you'll have it just a click away whenever you need it