According to www.w3schools.com/browsers/browsers_display.asp most computers are using a screen size of 1024x768 pixels or more. They also state that 1% of the computers have a screen resolution of 800x600.
At browsersize.googlelabs.com they are less optimistic. 30% of the users have a browsersize (that's different than the screensize) that is less than (estimated) 1020x570 pixels.
www.w3counter.com/globalstats.php states that 3.51 of the computers have a screen resolution of 800x600 pixels.
In July 2006 Jacob Nielsen wrote in an alertbox about screen resolution that you should Optimize Web pages for 1024x768, but use a liquid layout that stretches well for any resolution, from 800x600 to 1280x1024.
At Browser News they state that Resolutions vary a great deal. Most users have 1024x768 or higher, but a large minority have less. It is important to note that (a) the display resolution says little about the size of the browser window, and (b) users can normally resize the browser window. Consequently no particular browser window size should be assumed.
I want to repeat that last statement: no particular browser window size should be assumed
However for testing the sites I have to decide what screen sizes I will use to test. Normally I have a 1024x769 monitor and all sites seems to be okay their. I assume that that will also be the case for higher resolutions. I think that I will test the sites in the following resolutions:
- 800x600 for the old desktop browsers
- 480x800 for modern smartphones like the Nexus One
- 320x480 for the popular iPhone
If you can make it there, you can make it everywhere...