futura 111 wood -- to increase the top speed
 

1) generally , to increase the top speed should decrease or increase the fin?
2)with 7,2 mq is correct a fin 42 cm ?
Weight 67 kg, flat water..

thanks

42 cm is better suited to the lower range of your 7.2 sqm. A 36 would give you more speed in well powered conditions. If the water is really flat you can try going down to ~32 but this requires some practice and you may experience many spin-outs, notably upwind. However you may consider this set-up if you're looking for max speed. Some sailors I know use what seems outrageaously small fins for their sails.