Due to less drag of the board, you can expect to use around 20-25% smaller kite compare to other kind of boards.
Here a short shoot in the La Baule KTE last Week end (last Saturday in the morning),
I was riding a 14m Notus with 4m extension lines (total of 28m line length)
The only raceboards on the water were riding 17m and were underpowered. I was full powered!
About the size of the kite in term of maximum size, it depends on your target.
My 2nd most used kite with a foil is 21m kite size, like this I'm full powered with around 5knots (going upwing, downwind, with the kite steady in the wind window)
Nicolashttp://www.kiteforum.tv/index.php?optio ... baule_2012