The problem with using much more propeller than a 10 x 6 is RPM. The .40LA should be propped to turn in the high 11,000's. Many propellers used today are suggested by the manufacturers for noise reduction, but don't always allow the engine to develop good power.

Depending upon the trainer, you'll need a different prop. However, get the engine turning up in its best RPM range and THEN choose the prop that allows the engine to turn up AND fly your model well. If it's an 11 x 5, a 10 x 6, a 10 x 5 or a 9 x 7 or 8...who cares? It's how the plane flies when the engine's running correctly.