Small font size.
Font size is a very important element on advertisement and even though the choice of size of fonts can vary depending on the advertiser's intentions, it would certainly be counterproductive to choose something small. Small fonts are used for many other purposes for example, sometimes advertisers have to adhere to regulations and include certain information that might not be so convenient for their selling purposes; for example in the case of a brand of beer, you might have noticed that they do include a message about moderation on their publicity but its on the smallest least appealing font.
If there is something you and advertiser wants to make stand out they should be considering other resources like big fonts, very alive colors, placement, and attractive images.