Last year I used 7.5m V8 on my F111 with a Select SL7 39cm. It worked real well and I would not go bigger. This year I'm using 7.0m RSS MkIV and have not had much wind to try it out and sort out which fin I should use.
Any where between 7.0m and 7.5m would suit. Myself I would recommend a 2 or 3 cambered sail in these sizes.