I noticed this post over at the Adsense blog. adsense.blogspot.com/2008/11/optimisation-essentials-part-i.html Quote from article is below; Publishers often ask us what the best ad sizes are to include in their site's design, and we always point them to these three: 300x250 medium rectangle 728x90 leaderboard 160x600 skyscraper These ad units have proven in the past to deliver better results for both publishers and advertisers. Advertisers favour these formats, and if you've receiving all ad types, you'll find that advertisers will specifically target your sites more often. If you position your ad units well, users will be more likely to see these ad formats and find an ad that they're interested in. Their is a similar thread here on DP that mentions their success with bigger size ads. That 300 X 250 seems to be working well for the publisher.