Which are the ad sizes supported for HTML5 ads?
HTML5 ads are supported only on Web Inventories and here is the list of the recommended ad sizes:
300 X 250 Medium Rectangle
300 X 600 Half Page
160 X 600 Wide Skyscraper
970 X 250 Billboard
728 X 90 Leaderboard
320 X 50 Mobile Leaderboard
For further information about the HTML5 ad requirements and to learn more, check out Rich Media Ads.