RE: what fin size for a 6.8m on a S type104
Well, yes - but mainly as you'll most likely simply never get it going fast enough in 6.8 conditions (except speed trial) to generate enough lift to offset the sail/rider load.
Sailing the ST's in marginal conditions (for their board/sail size combo/s) you do need to rely more on the fin (size/area) due to the softer (bevelled) rail on the ST's (c/w say iSonic in same size). Of course, that ST bevel helps a lot at speed, in mid to high end and choppy conditions, but in marginal 6.8m/ST104/85kg conditions you need some more fin than 29cm.
Obviously that extra fin size/area helps the ST stay upwind (more efficently than dropping the windward rail) in marginal conditions too.
Cheers ~ Ian