Screen Resolution Compability
Make sure your website is compatible with the most relevant screen resolutions with this tool from Google. It overlays your website with an image outlining screen resolutions and their relative share.
Official links for Screen Resolution Compability
Screen Resolution Compability Review
We recommend building websites that are at least compatible to 95% of the user's screen resolutions. That's 800x400.
Note that the Google Tool actually uses the client window width as basis. That is important since the browser's toolbar and controls also take up space.
See an example of the Google Browser sizes tool with SEO-Tools.mobi loaded:
The Google Tool is still on Google Labs. However it's pretty useful. One downside is that it does not really work with centered designs (such as ours). If your design is centered, you have to resize your browser so that your website aligns left.
If you want your website to be compatible to any screen resolution (e.g. mobile devices), you have to defined positions and sizes relative (in EM). However that is more a design than SEO topic, and therefore will not be detailed here.