Nice new addition....
We're now happy to announce the launch of a related feature you've been asking for -- 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.
Inside AdSense: Font size matters
In case you don't read the whole thing...this part is important:
Also, a bit of advance notice: soon, we'll also be changing the default font size across AdSense in ad units from 'small' to 'medium'. We've heard from some of you that the current font size can be too small, and our testing also showed that slightly larger font sizes improved performance. Any ad units set to the default font size will automatically be updated to display the larger setting. If you'd prefer to stick with today's font size, you can set your desired font size as 'small' across your account.