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I agree that bigger is better, but not in every case. Google has stated and continues to state that the 3 formats that perform the best are: 728×90, 300×250, and 160×600. They mean business with this too, because they are not letting up about informing publishers of this.
However, I have seen some smaller formats outperform some of my bigger banners time and time again.
Something I’d like to remind everyone is to take what Google says with a grain of salt. Do not take what Google says as the only and last authority on the subject. They are just one resource out of thousands of resources out there that can help and guide you as a publisher.
A couple years ago, Christian Ashlock wrote an article sharing some tips about how to optimize your AdSense earnings through different types of advertising. I think this article is still relevant today that it’s not just size, but it’s also type.
Take a look at the article, I think it’s a worthwhile read.