Not from the team but from my experience..this is a very difficoult question and there are many aspects that are involved, and I kinda share mark's thinking..
I'm sailing iS 75cm for 3 years now and all that I can say is: in flat, little choppy water I (72kgs) like to sail 9 till 20 and 7.7 from 20 to maybe 26-7. I was on water beyond these limits with both sail sizes but it become a bit unconfortable. It also depends if you're freesailing or downwind or on the reach, etc..
With more chop this limits goes down but I can't really say: ehi with 17kt of wind and 50cm chop the best is XY because it's subjective and depends on where you're sailing/how you like to sail/which type of sails you have/which model etc..
Not much shortcuts here, except for the excellent fin-sail size chart that Remi gave you, just have to find your best combo..