as for kite sizes for foil kites...2 views..
1. Get 2 kites without much overlap so if you have a limited budget you can buy the premium light weight kites and have a massive wind range. here we are assuming conditions are suitable for say 5-25 knot range
2. get 2 kites with lot of overlap and you are trying to cover the sweet spot for the location and hydrofoil wing with option to use either kite if the other is in repair or not with you on beach. If you look at all the sports and how people progress you can notice that most will eventually specialise and prefer a certain narrow range of equipment, conditions and friends . Initially you cast the net wide and slowly you are not interested in say 10kts an door boards or freeride hydrofoil. You may decide to go full race or full wave with kite like Peak 4. if the conditions are less than ideal you may enjoy playstation to kiting. Accessible Hydrofoiling is new so people forget that it's a matter of time till they stop getting that buzz out of every session