I find that a smaller board is actually better for riding a big wing in light winds.
I ride my 250 sqin NP Glide L foil in 7-15 knots with my Peak 4 5m. The key to water starting it in 8 knots is being able to sink the board while standing on top of it. I use a Slingshot DC 4-6 board and use 1-2 loops just to drive the whole kit under water (need head deep water for this). Then you're just standing on top of the board thigh deep in the water and you do 1-3 more loops to get the rig moving laterally and generating lift to come up out of then water then on to the foil.