Personally I'd be riding that as a quad or 2+1.
But if you wanted to go a single I'd say something 7-7.5 inch with a fair bit of base.
General rule is short, stiff upright fins back in the box for pivot or more flex, raked fins further up on the box to draw out your turns.
A good starting point is to lay it flat on the board with the bottom of the fin on the stringer and slide it up till a third of the fin is hanging over the rail than stand it up and should be pretty close to the right spot for that fin.
Something like this in a 7 might be OK.