RE: what fin size for a 6.8m on a S type104
Considering the rocker and basic layout of the ST104 is similar to the S100/S95 - except with even less "edge" on the ST because of the bevel, then it's logical that the ST follows similar to Sonic fin tuning, perhaps even a bit more fin on ST due to the bevel edge/increase width etc. OK, not identical but you get the idea. In "typical" 6.8 conditions (especially marginal 6.8m conditions) a bigger fin than the stock 29cm would offer some advantage (in general 6.8m sailing use).
It's also previous solid feedback from many ST customer users that they consider ST's do perform better in lighter conditions (bigger sail sizes) with more fin than specced.
BTW, did you see the ST104 fin spec for 2007 ?
Cheers ~ Ian