https://www.engineeringtoolbox.com/cast-iron-soil-pipe-dimensions-d_1751.html
I don't do much with cast iron, but I'm going to say that hub is 2" cast. See chart linked.
2" steel or PVC is 2-3/8" OD.
I don't know about Ferncos, but I'm guessing that you need a 2" x 2".