Just found this pretty neat feature from Google Labs - a web 'mask' showing usage percentages for browser window sizes.
Pretty useful tool for us website designers! I'd say it looks like optimising your website design for c. 1000x550px resolution is a happy medium (at the moment) of usability over design possibilities... Although some would say you should make your site available to absolutely everyone!
There's also useful raw chronological browser stats including size, operating system, browser, from W3 Schools http://www.w3schools.com/browsers/browsers_display.asp