21 BFK is big.

At our size, IMO, it's overkill. We are light enough it may not be necessary.
No exaggeration here, I can easily ride a 15m on the Glide in 9mph. In 11, at our size, I am having quite a bit of fun.

Less than 9, I don't know that a bigger kite is the answer, anyways. Wind is not linear in power, so every knot drop is a HUGE drop in power. Its pretty strange.

Sounds like you are moderately new to the sport. I would play it safer than sorry. Maybe right around 17m.

Yeah, try to test ride one (Glide) , you'll see first hand what I am saying. It's really quite amazing. 8)
I have seen first hand maybe 20 people all claim the wind picked up.. good stuff.