It's 1366 x 768, a pretty popular screen resolution for laptops. You can see for yourself here:

http://marketshare.hitslink.com/report.aspx?qprid=17&qptimeframe=Y

It might be a better idea to specify font sizes as a percentage, and line heights in em. I believe the layout will adapt better to different screen resolutions. It's really important for things to look good on, at least, the most popular screen sizes.