I have gone ahead and ran a few tests. It seems that the medium font size performs the best. By the way, what's up with people using Times New Roman? I know it's kind of off topic, but it's such a hard font to read. Anyway, the reason the Large font doesn't work as well is likely because it's to squished together.
Not really ! Medium sized fonts are good for a ad. Making them big won't help you much and also that won't look good in your blog/site. - Digital -
You really need to test on them. I cant see much different but medium size is providing abit better CTR compare to the other sizes.
There was about a 20% increase on the sites that I selected to test it out on, and it hasn't gone down in the almost a month that I've been testing it. I'm considering re-designing if it maintains that increase.
my opinion to make the ads blend in with the rest of the page, too big and they will look like billboards