Fuel - You can try your dad's 9m, it will give you that tiny bit more power and pull, and give you more of an idea the pull you will get fmr the 11m.
I really think for you to get the best out of the summer kiting season if you want to get out in 15/16->24ish knots, grab an 11m kite for these conditions.
Ask a store for a demo or one of your kiting mates to have a shot on their 10/11/12m kite, and see how you like the power and pull

You will be plesently surprised

Goodwinds
Elizabeth Bate - Wello Kiter