I had a 12, 9 (Lithiums) and 7 (Crossbow) for quite a while and found I was hardly using the 12 (with a Nugget) but was overpowered on the 9 above 30 knots.
Now I have a 10 and 8 (Airush Unions) and use them most of the time. Good de-power means you get better wind range, so you need to research the kite characteristics and not just consider the size.
I also have an Ocean Rodeo Flite 14.5 which gets me out in very light winds, but not too often. Its a great kite for its size.
Have put some more info on quivers here:
kitesurfing-handbook.peterskiteboarding.com/gear/kite-quivers