I did a quick search online and found this excellent page on a pixel ruler for Windows computers:

Here is a ruler in pixels:

and here is the website where you can download the ruler for your computer:

http://www.arulerforwindows.com

I do hope this helps :-)

If you'd like to download a nifty little converter, pop over to this page for both Mac & Windows download >>

And to put this in clear terms:

500 x 300 pixels

= 5.21 x 3.13 inches

= multiply inches by 2.65 to find out centimetres = 8.0234 x 7.9502 cms