Here is a screen shot of the three sizes you can pick:
Google has been testing both larger fonts sizes and smaller ones here and there, so this was expected, and it is now here.
One important note that AdSenseAdvisor said in the thread:
I also wanted to give you a heads up that soon we'll be setting the default font size a little bit bigger based on your feedback that sometimes the text ads on your page are hard to read. I'll keep you posted.
Forum discussion at WebmasterWorld.