Best Performing Ad Formats, Ad Sizes

Phoebe Ho of Google AdSense's Optimization team reports in a Google AdSense Optimization Webinar, that the best performing ad size is the large rectangle, 336x280. She also stresses the importance of using the best format for your own site, though: 

"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.

As I had mentioned before, because there's a lot of variety with different sites, you want to make sure that you're using the best format for your site. Don't try to squeeze in a format that doesn't fit your site just because we said so here today. I want to make sure that you fit in the right format that would fit into your site. But just to let you know that wider formats do tend to perform better."

Google is not alone in this observation, either. For example, in Best Practices for Optimizing Web Advertising Effectiveness (PDF), based on a multitude of statistical studies that “show a strong correlation between the size of ads and their click-through performance”, the authors recommend larger ad sizes, especially the 336x280 size, format: “measuring click-through rate (CTR) and purchase intent respectively, the “large rectangle”(336x280 pixels) was particularly noteworthy in its effectiveness compared to other standard on-page display ad units.” The publication also mentions that the 728x90 leaderboard format is gaining in use, too, as being a desirable ad size.