birdstrike,
If you opt to build a trainer, all of the previous comments have merit. That said, I have a box with trainers in it. It has a "Brodak Basic Trainer" powered by a OK Cub .049 engine, a "SIG Skyray" powered by a Cox Black Widow, and a couple of sheet wing planes similar to the Skyray, Cox powered.
A few pluses for the .049 size trainers: Don't need a large space to fly, easy to build and repair, nearly indestructible (if flown off grass).
To me the most important item is that the controls are mounted externally so they can be adjusted from small movement, used until you stop getting dizzy going round and round (and you will!) to more aggressive control movement allowing you to do stunts.
I would suggest 15% nitro, 20% lube fuel (make sure some of the lube is castor. You may wish to use Dacron or monofilament lines at first for easy handling, but you should eventually go to .008 braided steel lines for better performance. Some people misjudge 1/2A because they do not set up for best performance.

What size trainer you use might be dictated by what engines you have on hand. General rule of thumb for balsa planes is that smaller planes are harder to break.
Many have professed the good points of getting a foamie as a trainer. Your choice.
Whatever you get, I recommend an external control system so you can adjust the throws.

George