To me the sweet spot kite size for foiling is 1/2 the size of the surf kiters and down to 1/3 the size of TT. I can get away with going smaller but I normally only do that when the forecast is for the wind to pick up. I generally prefer to have a bit of power to play with. Power adds stability when things get a bit sketchy.
As I've said in another thread, I also find I can go the same size kite as the surf and TT kiters. It's more fun having the "right" size kite, but it's very doable to have a big kite, especially if the waves are small.