I know that the default font for Adsense is supposed to be Arial. I have never changed my default font family/size. Starting around June 15 ads on my website started using what looks like the Verdana font. Titles are bigger and CTR is up a bit. Is Verdana the new default font?
I should have mentioned I was mainly inquiring about skyscraper ads. Researching this a bit more I see that Google is showing all kinds of font sizes depending on ad format. For example, large 300x250 rectangles seem to generally display bigger fonts than skycrapers. Smaller 460x60 banner ads have smaller titles. So I guess that default now means whatever font size/type they deem more profitable.