My reference in termos of wind are the following:
I use a 8.5 between 10 and 20 knots
I use a 7.0 between 15 and 25 knots
I use a 6.2 between 20 and 30 knots
I use a 5.2 around 30 knots
I use a 4.7 with more than 30knots
I think as i dont have a good tecnhique i have to use bigger sails to get going.....
I´m 80kgs but again my spot, depending on the tide, you can have flat water with high wind or very choppy water!!!