Depends on what you mean by trainer; Basic trainer or Stunt trainer?

If you mean a basic trainer, then I vote for the Guillows Basic Trainer but they are impossible to find these days. Next vote is the Brodak Flight Streak Trainer, with sheet balsa wing, fuselage and tail.

But the cheapest and easiest way to learn to fly is an Osborne Platter with a Cox .049 on the nose. Extremely stable, easy to fly, handles wind. Cheap and quick to build and nearly indestructible! With a few minor mods it will even serve as a great stunt trainer that will survive many many crashes without serious damage.

I once built 3 platters for a day of teaching kids to fly while the grown ups participated in a Giant Scale Fly In. I thought I would need 3 complete airframes with engines to get me through the day. The first platter lasted all day and flew countless flights which almost always ended in a crash, only occasionally needing a drop or two of CA before the next flight. Went through a lot of props that day!

Even the RC guys stood inline with the kids to fly it!

Sorry but all of these are smaller than .35 to .60. Most planes in the size you requested will be designed for stunt not for basic training. For stunt training in that size I would think about a Sig Sky Ray, S1 Ringmaster or similar profile non-flapped stunt plane.

The ARF Flight Streak is pretty fast and somewhat fragile, the ARF Nobler is a great flyer but also fragile. Seems to me you want a high level of durability with a trainer.