I wanted to say 7.5 but I wanted people to like me.For what it's worth, I posted in the other recent thread, I scored a used 8'2" Magic that came with a 7.5" Skip Frye Fin. From looking at it, it sure appeared to be under finned. The thing just flat out works. As Smukes noted, it's all about the double concave. Years ago I saw a Mitsven Egg, IIRC, at Mollusk SF, back then it appeared under finned as well. The shop employee mentioned that Bob loves deep concaves and that allows him to use a smaller fin.
I ride mine in small to medium size beach break, OBSF, and it works great.