I have a Futura 121 and a Severne NCX 7.5 sail. I live currently in Thailand where the winds this season are generally light, 6 m/s or less (12 knots) and I find the 7.5 sail too small for planing. I would go for a 8 or 8.5 sail if I was to choose today. For days with more wind, perhaps 6 or 6.5.
I am 184 cm and 74 kg.