Hi Heli, I use and sell the Naish gear, Im 76kg been using the Hover 122/Thrust WS for 12 months. The Lift 5.7 gets me going in 10-12 knots, once up and foiling, I can glide through some very light lulls, lots of low down power with the 'slot' closed.
My go-to sail is the Lift 4.7, but the 5.7 works when it has to. If you are bigger than me, or want to push the light wind window, the 6.4 should work well for you. ( I am learning to sail the 5.7 overpowered if caught out, simply flattened out, and lots of de-powering with back hand! Going really broad down wind works with the back hand bought forward to the mast, sail just feathering, just really flying on the foil.....such a blast)
Cheers