I would go about a 12X6 or so Master Airscrew, I say Master Airscrew because you will get a few prop strikes here and there while you are learning and the MAS props can take a beating.
Assemble it as a trike for starters and it can easily be modified later. The Kadet Senior is by far one of the best trainers around.
Funny this thread came up. My kids bought me a Kadet Senior ARF for my birthday, just arrived a couple of hours ago.
I traded my old one some time ago and am quite happy to have another one.