As Crazychemical said, tuning is quite a personal thing, so you have to find what works for you.
I like riding my S-Type 93 well powered up and on the fin, so the fin has to be powerful enough, but also "slippery fast" and reliable at the same time. The 34 cm Drake Slalom (SR6B replica) that came with my HS96 turned out to be a pretty good fin for it for larger sails sizes (5.8-6.8). I recently got some C3 Venom fins and the 32 has been sweet with a 5.8 sail. I use Lessacher LeWo speed fins for 5.0 (27 or 30) and switch to a smaller board below that.
I remember Ian Fox saying that the S-Type 93 is probably at its best near its maximum sail size and well powered up. I sort of ignored that for a long time, remembering how much better 6.8 was on bigger boards. A few weeks ago, I tried 6.8 on the S-Type 93 and was very positively surprised, so I'll re-visit that combination when the wind permits (maybe later today).
The ST93 is turning out to be one of my all time favorite boards.