RE: S-Type 93 or 104?
I use Select Supercross fins in size of 32cm and 26cm. The 32cm is fine with the 6,6.
The 26cm is great with the 4,7 and still ok with the 5,6.
If your smaller sail is around 5,5 , a 28cm would be perfect.
My 6,6 is a 0 cam freerace sail (Loft O2) which is quite light relative to its size.
It matches well with the ST-93 when blasting on flat water in 15/20kts wind, but I would not go bigger in sail size.
The ST-93 is very well balanced with the 5,6 in more choppy open water.
The upgrade with the two new fins has really open the riding range.