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.
Last edited by Farlo; 27th September 2009 at 10:31 PM.