Best Performing Ad Sizes, Ad Formats

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.