I lived on the Big Island for a couple years, did less kitesurfing than any other period of my life. A-bay is probably the best place to go. It is alright, but definitely not worth traveling for. Like pretty much every other coast of the big island, it is brutal. Sharp lava rock instead of sand. You don't want to be stuck walking in at low tide (happened to me when the wind died, my feet were in really bad shape afterwards). In general, there isn't very good wind. A 12m kite would probably be the standard size for a smaller guy, and don't expect to kite every day. There is a local crew that is really helpful, I strongly recommend talking to them before going out. A lot of people think that since it is so close to Maui, it must be at least almost as good, and this couldn't be farther from the truth.
I'll sum it up like this: kitesurfing is my single favorite sport, but next time I head back to the Big Island, I'm not going to bring kiting gear. World class diving, wonderful snorkeling, amazing hiking, good birding (in a few spots), volcanos, cloud forests. You can get a handful of mediocre kiting sessions in, but you'll probably have a better time doing something that the Big Island does really well (which is a lot). If you want to kite, go to Maui, or Oahu, or Kauai. You can kite on the Big Island, but it really isn't worth traveling for, and if you find yourself there, you can probably find something better to do. If you live there, it is very different- there is ok kitesurfing, but it just isn't really worth traveling for, in my opinion.