Google AdSense has just added a brand new 200×200 “Small Square” ad unit size to their regular ad unit line up. So new, in fact, that AdSense hasn’t announced it yet, and it hasn’t yet made it into their ad format page yet.
Here is an example of the ad unit:
It is available for both text ads and images ads, so it is very likely that they could also be utilizing them for videos as well.
You will find it in your AdSense control panel in the drop down menu when selecting your ad unit size for a new ad.