Hi there, really depends a bit in foiling on your skill levels, the better you become, the more efficient you are in pumping/using the foil to create power. So normally you should be able to start with a powerful foil sail of 7.0 already in 10knots, using a wave sail around between 4.0-4.5 in 25knots....but I often see people windfoiling with 5.3 Freestyle sails as their biggest sails in 10-15knots, they manage to pump up onto the foil very early. I think with some skill even a 6.4 should be enough for you in 10knots, but again, really depends on the skill level - smaller sails are of course lighter and easier to pump, especially wavesails or no-cam sails can be easier. Cammed specific Foil sails of course is a different story, as they create a lot of lift/are generally softer for pumping.
Confused? Hope not, just saying it does depend a lot on skill level, type of sails.