There's a thread in the 1/2A forum talking about a .049 trainer. Don't know how many channels they are discussing, but 4 is certainly used in 1/2A. I've also come across .15 size trainers on several occasions. personally, .15 is the smallest I would go. With this size, the engine will have all the reliability and ease of operation as its bigger brother (the .049s can be finicky), but the plane will be nice and small. You will also probably get a number of people tellling you that the only "proper" size for learning is the traditional .40 size trainer. While it's true that a larger plane will do better in the wind, planes in the .15 size range do better than most of them realize.