I would recommend going out whenever the kite can relaunch easily. Probably somewhere between 12 and 16 knots.
In the beginning you will prefer more power but for most people that will rapidly turn around to less power.
Regarding kite size: it might depend a bit on the type/size of the foil but I think a 10m will get you down to 11 knots without much problems once you can ride. Probably still good to 15 knots but it depends a bit on how powered you like to be on the foil. For me it would be at the upper end of what I like (it’s certainly possible to ride with lots of power but many foilers prefer small kites, low power. IMHO It makes the session more playful since you can throw your kite around without worrying about getting yanked of the board. A smaller kite is also safer especially with the foil itself already trying to kill you)