In addition to font faces, Google now allows of changing font size of the text in your AdSense ad units. You can select from small, medium, and large font sizes for ad units on pages in Latin-character languages. The font size you choose will be applied to the body of the ad, with the title scaled appropriately. In order to fit the ad text correctly, the actual font size will vary for each format size, font face, and user-specific settings such as browsers and operating systems. You can select a new font size on an account-wide basis in the Ad Display Preference section of your My Account tab, or on an individual ad unit basis for new and existing ad units.

More Info: AdSense Blog