Interesting what you say. My understanding was that a lighter rider would need a smaller fin, all other things being equal: wind speed, sea state, sail size, but... Christine le Couturier (French SB team) revealed that she uses the same sail/fin combinations than her 20 Kgs heavier boyfriend, but in lighter conditions.
Also speedseekers manage to use fins significantly smaller that you would expect for their weight and sails. For instance on my lake the speed record was set by a 90 Kgs guy with 8.4 mē and "only" 36 cm fin. The same day I was also having a 36 but with a 7.3 (well overpowered).
To choose a fin I use a slightly different rule: sail size x5.5 in ideal conditions, x5 when overpovered and x6 when underpowered. This combined with some outhaul extends greatly the range of a sail. This works for my weight (68 Kgs) and freeride/freerace, not slalom/speed/competition. Hope this helps.
Last edited by Farlo; 25th January 2010 at 08:30 PM.