What does finlenght do? I know one thing for sure, finlenght does create lift. Larger fins are easier to bend, combined with an larger leverage. This causes significant more lift when fins get bigger.
More finlenght can give the fin more power, however more finlenght does require more downforce, or the tip of the fin should be able to twist... But when the fin is able to twist you will lose power you just wanted.
Why do people go faster with smaller fins??? This one is not so hard, with smaller fins you can use smaller boards. Since the drag from the boards is quite high the smaller fin will get the efficiency up by using smaller board. BUT don't forget larger fins can be sailed very fast, only they haven't won (yet?!?) from the smaller boards with powerfull fins.
I looked on www.gps-speedsurfing.com a while ago and this are the stats of Finlenght vs Speed:
Speed on a windsurfer, it is not only about finlenght for sure. There are just a lot of ways that we (windsurfers) have not tryed. For now, it is just to get an as powerfull fin as possible under the right board. The right board is small, but not to small. Don't forget wider boards are a lot of times faster...
Maybe in the future there will be wider shapes with higher efficiency... This would be the easy way (if possible) to break records/ win competition. One of the wider boards with high efficiency is the Fanatic Falcon series... But there should be more solutions to get bigger leverage vs less boarddrag.
One thing I know for sure: My new select SL7 (2008) are giving a lot of power! Combined with the very low drag Carbon Art Speed boards...