Good one.

Here's what I saw and heard in St. Augustine today.
We had light winds (get a 4 foot jump in during a gust kinda day or around 9-12 knots). Shannon Best was on a 16m Waroo. He was doing 4 foot tricks including handle passes. He wasn't on a 20m. Not sure if he had one or not. He rode for about 45 minutes in the afternoon.
I asked him about the 20m Waroo. Specifically, the top end on the kite. He wasn't sure. So, given what I have seen with him pulling off handle passes and Jimmy Hill (big, strong guy) having a great time in 12 to 14, I think the 16m Waroo is probably as big as kite as you will need.
At 160 pounds myself, I am pretty powered on that kite in solid 10knots.
IMO, the board is gonna make or break you in these ultra light winds anyway.
Daryl actually rides a 14m Naish Torch as his biggest kite here in St. Augustine and Pat only goes up to a 16m RRD HIR (those are both C kites). They both hop on SS Glides in light wind.