about .60 size........

The engine size is not really important in my opinion for a trainer.
A trainer must be as stable and big as possible.
the design of the structure is important, if light it make less drag, fly slower and doesn't require big engine.
the kadet senior (kit version) for exemple is a impressive 78' inches wingspan bird, a excelent trainer and it fly great with a .25 two stroke when correctly build. Low speed characteristic (the most important parameter for a trainer to me) are awesome.

If you are ready to build a all sticks airframe jump to the kadet senior. he has no ailerons, but really doesn't need it.