My favorate prop for a trainer with an OS .40LA was the APC 10x5. I ran that engine/prop combo on a beater trainer for quite a while. (also, you can take the baffle out of the muffler for free power at the expense of a little extra noise).

CG's explination is really good, but keep in mind that the only way to pick a prop for a particular plane is to go get one and try it. Airframe drag plays a huge part of how a particular prop works on a particular engine in flight. This is why RPM measurements on the ground don't always tell the whole story.

Also, glow engines are not linear. Sometimes you can actually go faster in level flight by dropping pitch and not changing the diameter. The reason is that the lower pitch prop will load the engine less and you will get more RPM. Depending on the torque curve of the engine and other factors you might just get enough extra RPM to more than make up for the loss in pitch. Sometimes you can go faster by keeping the same pitch and going up in diameter, though that is less common. (if that happens to you, then you probably have a draggy airframe and were trying to prop it like a pylon racer).

One last thing, if you change props and don't notice an RPM change on the ground, that should tell you something. If you went from a larger prop to a smaller one, and the RPM didn't go up, you're probably redlining the engine already, and you should go back to the larger prop. If you put on a bigger prop, and the RPM didn't go down at least a little, then the first prop is almost certainly too small. At the very least, you aren't getting the most from the engine.