RE: iS133 fin sizes
For iSonic 133, I've found the stock 48 cm fin to be pretty optimal when well to overpowered with my 10.6 sail. Once underpowered or for earliest planing, a 54 cm fin is better.
With a 9.5 sail, I would say a 48 cm fin is spot on. If you really want early planing in lowest possible wind, I would go up to 50-52 cm max, no more, otherwise drag starts to kick in and will limit the top speed with your sail.