Google AdSense has introduced the ability to change the font size of the text in your ad units. You can now select from small, medium, and large font sizes for ad units on pages in Latin-character languages. Just like with the current font face options, size options appear in every account but will only be applied to ad units on pages primarily in Latin-based characters for now.
The font size you choose will be applied to the body of the ad, with the title scaled appropriately. More details can be found at Official Google Blog: