Roseanne van Langenberg of Marketing Defined not only says she is getting more stickiness and better Adsense CTR from small size fonts - her site is completely created with tiny fonts (the adsense ad fonts are slightly bigger though). She says she's doubled Adsense CTR and now averages 3 page views per visitor.
I must say that the font is very small, and while I find reading online tiring anyway, her pages (I assume a 4 pt font or smaller) take concentration to read. I guess that forces one to slow down and read more because it's virtually impossible to skim.
One point to be made, though, is that Roseanne also made some ad placement changes and other page changes, so while the CTR doubled it's not absolutely clear that is what made all the difference. But if you want to test it out by all means, it's worth a try. I will try this techique on one of my sites using adsense and report back here.
All this was based on the Eyetrack III study that came to some very interesting conclusions, including utilizing smaller fonts. It's been a trend lately in web design, but if everyone takes this advice, my online reading will be cut down noticeably or I will have massive migraines ;^)
PS. Just for clarity sake, I have my browser text size set to "medium" - on IE go to "view" then "text size" and pull down to the setting you prefer. Might have to start setting it on Largest one of these days.