Check from the hydrofoil forum,there are many threads about this. These are the most recent
viewtopic.php?f=196&t=2399787
viewtopic.php?f=196&t=2400024
viewtopic.php?f=196&t=2399814
Right answer can be 12-18m. It depends of the conditions you kite and the kiting style you want to pursue.
Kiting style:
Strapless freestyle => go with 12 or 15m
freeride with straps => go with 15m
Big air => go with 18m
Racing => go with race kite in size 18
Conditions:
How bad lulls you have? If the wind completely dies and tube kite drops when laying still on the water. Then you want 18m. It reacts slower, also to lulls. It has more energy you can use when the lull comes by pulling the bar.
More constant wind => 15m most likely sufficient.
What you mean with 5-9 knots. If you see people kiting with doors in these conditions, then 12m is sufficient. If you want to kite when everyone else is watching, then 15-18m is needed.
Why not 18m. 18m has much worse high end and gust handling ability. You can fight this with short lines.
In my local lake we have 2 frequent hydrofoil kiters. Both of us would use 18m sonic 2 / sonic race in those conditions. But it is just our kiting style (big air /racing)