When you say 15-25 knots do you mean average? Or do you mean gusting to 25 with lulls at 15? I’m around 77 Kg and I wouldn’t dream of being on a 122 liter board with a 25 knot average, more like a 70-85 liter board.
If you are talking about gusts to 25 with lulls to 15, then I would say yes to the 7.6m sail. If you are talking about averages from 15 to 25 then I would say a 6.5m sail, but on a much smaller board like the Futura 93.