What's you first kite's size and make?
It's a good idea to have a continuous quiver..
Like, if you have a 12, then definitely go with 17 (3 sizes up is not enough to make much of the difference for low wind)
If you have a 9, then probably 15..
Since you are interested in boosting in low wind, you probably want relatively big door and BIG kite.. don't expect in miracles with TT, but 5m should be doable with 17m kite in 14kts.
Once you get to foiling, you can easily double that height with even less wind.. will still need a big kite though.. my favorite setup for 8-12kts is Ozone Zephyr 17 and my freerace foil wing.
I can get decent airs while TT riders barely keep their ground (if they are lucky)