Windsurfing & Stand-Up Paddleboarding in Michigan
Not a fin expert, but I do have more fins than I care to admit to. I think the 46 cm should be good with your 9.5 m2 sail, depending on the board and depending on how efficient you are. You mentioned two different boards. There's one rule of thumb that says if you measure the board width one foot from the tail, that's the fin size you need. I don't know how accurate that is, but wider boards do need larger fins. There's a fin size calculator here that might serve as a guide:
If you're sailing a 5.5 on that big starboard, then you might need a smaller fin, but only if you're feeling overpowered. Most of use would be on much smaller board with a 5.5 m2 sail.