I got Franks on a Worksman INB with Wald 962 fenders. There's a ton of clearance between the fenders and the rubber; the limiting factor in my situation lies between the seatstays.... I don't think I can go much fatter with the tires without them rubbing the seatstays....
What I'm trying to say is, there's a lot of variables here. Beyond the tires, we need to consider which frame, which fenders, and even which rims we're using. Also, FWIW, the Franks tend to be pretty close to true size, whereas a lot of cheaper (low-buck Kendas, CSTs, Sunlite) cruiser tires might have the 2.125" label, but they're running closer to 2.0". Regardless, if you have a balloon bike, it'll probably fite fenders and ranks and, really, do you want to be running fenders that are out of alignment anyway?