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SA_AL said..
As I watch Naish going with a very small sail (~5m) for wind of less than 10 mph on his foil board and racer using 9-10 meter sails with foil racing, I have no idea what I use for low wind days. I prefer to use smaller sail if I can foil efficiently as a beginner. So far, I was able to manage 12-15 mph with 7.0 on i99 foil but I am wondering using much bigger sail will give me a better flight. I predict that recreational foiling is significantly slower compared to the race foiling but can I increase my lifts using much bigger sails? On a separate note that as I watch racers on foil, it seems they are able to ride with an angle pushing their board and fin from the side of the board while recreational sailors are more straight.
It all depends on the foil/board combinations. Racers use 160-180 liters boards, 100-110 fuselage and relatively small surface wings. Add the width of 91cm, with a tail almost as wide, and you need a 8.0-9.0 in light air to start going (the drag induced by the width is huge). And yes, race board/foils are designed to go upwind and downwind. (And no, nobody had a 10 in the PWA last year.)
Take a Freeride foil with a shorter 80-90 cm fuselage, big big wing and a narrower 70-85 boards with comparatively narrower tails, and I have seen people taking off in ridiculously low winds with sails two square meters smaller than what was been used at the same time on a Starboard Foil 177/F4.
7.0 seems already big, and it might be that you have to work on your pumping technique? I am terrible at that and it seems to be what makes a difference in light wind with respect to other sailors who have mastered it.