Medium rectangles & leaderboards make up nearly 2/3 of the ads served on Google's ad networks

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Ad size & percent of ad impressions
"300 x 250 - 33%
728 x 90 - 32%
160 x 600 - 13%
468 x 60 - 3%
120 x 600 - 2%
300 x 100 - 2%
88 x 31 - 2%
300 x 600 - 1%
336 x 280 - 1%
320 x 50 (mobile banner) - 1%"

Source: Data from Google & Doubleclick, reported on page 25 of Display Business Trends - Publisher Edition, May 2012