Adsense has a feature to select the size of the font that appear on the website where it serves Adsense ads.
By default AdSense ads are set to the default Medium size. Basically AdSense ads use a default font family and font size which have performed well on the network as a whole. You may instead choose a preferred font family and size.
How to Change the Font and Font Size in Adsense
1. Click My Account
2. Click edit on the Ad Display Preference section( middle of My Account page )
3. Choose the right font and size
4. Click Save Changes
Font size and Font settings apply to all ads in your account unless you override them on individual ad units listed in Manage Ads.