The one engineer from the other website said it would have the largest tires Ford has ever put on a car.
So expect at least 325s, which would be good because there are decent tire options in that size for fairly cheap. (325/30 19)
I honestly don't see Ford retooling the quarter panel and as such, a 325 rear is about as big as I could see them going. And that is with a tire that has a similar rounded profile as a Cup2.