Read this transcipt if you put AdSense ads on your blog: Google AdSense employees discuss tips and tricks for new publishers.
"So the wider formats do best so specifically, the top three formats are the 336x280 that you see on the page; the 300x250 medium rectangle; and then the 160x600 wide skyscraper."
Also don't miss the info on color palettes, avoiding "ad blindness" and tracking.