Practice, practice, practice. Comfort only comes with time on the water. On your 10m, are you reaching or going upwind & downwind? Point of sail makes a big difference. Water state (chop and waves) also makes a difference. 18 knots in open water will be a lot more challenging than protected flat water, especially on the downwind runs.
Fin selection make a big difference too.
From my experience in racing formula (amateur), carring a large sail in strong winds is only an advantage on the downwind legs of the race. If you aren't racing, a 9m would be plenty of sail in 15 to 20 knots (8.5 in 18-20 knots) except for downwind.
An 11.0 would give you an advantage in 7-15 knots over a 10.0 (7-10 knots = getting on plane quicker, higher pointing & better downwind speed. 10-15 knots= better downwind speed).
I weigh 79kg and sail a F160 using 11.0, 9.3, 8.3 Maui Sails TR-3's and 4's. As I said, I am an amateur, plus I only race about 6 times a year. The top sailors carry much larger sails. I usually rig about 1.5m smaller that the top racers (my comfort range).