RE: Fin size to match 9.5 on iS133??
With a 9.5 sail, the stock 48 cm fin is spot on. If you want earliest planing in lowest possible wind, I would go for a 50-52 cm fin, no more. Rémi is using a SL2 52 cm fin with 10 m2 sail for low wind slalom racing (10-15 knots of wind if I recall accurately).
I've been using a 54 cm fin with a 10.6 sail for earliest planing as of 7 knots of wind with my 65 kg. In 9-12 knots of wind, the stock 48 cm fin is fine once well to overpowered with my 10.6 sail.