Hi Guys,
Yep, trainer kites are GOLD! When starting the sport, buy a trainer, they are good fun, will teach you intuitive flying and respect for power and everything else you need to know about the wind window.
Here's a thought though, most people I know, never sell their trainers and in fact still fly them on occassion, just check out the buy and sell to see what I mean. The reason is they sharpen reflexes, are safe, give you a good workout, are entertaining, especially when flying side by side with a mate who also has one

and are great to teach girlfriends, mates, and family without hooking them into your pride and joy, full sized kite. You can also ride them on a skateboard in a car park or on a mountainboard or buggy on the grass.
Here's my thoughts on trainer kite sizes. We sell mostly 2.5 and 3.5m 3-Line Trainers although we have them also in 1.5m, 2.0m and 3.0m. The larger they are, to a point, the more realistic their movement and power when considering they are training to you fly a large inflato or foil.
We sell 2 and 3 line trainers and by far the 3-line trainers are bought at a ratio of 10:1 against the 2 lines despite being up to 30% more expensive. Reason being, they are safer to land (they don't windmill upon bar release) and can be relaunched 100% of the time which you cannot do with the 2-lines. Many people stop flying their 2-lines because of the frustration of crashing and having to stop and reset for a launch. Backwards relaunch adds a whole new level of fun, skill set and useability to the trainer kite. Our trainer kites range in price from $160 to $325 and we sell a lot more in the upper price bracket. Cheap is not always best!
Just thoughts to consider when buying a trainer.
Good winds,