These web resources should help ya:
http://www.webstyleguide.com/page/dimensions.html
http://mdp.artcenter.edu/~vanallen/w...eb1_wk04a.html
The sizes do differ from browser to browser and OS to OS, so use the recommendations above with a pinch of salt. Whereever possible, avoid going right up to the pixel limit, and you should be just fine...