"best fit" is what we all wish for. The best we can hope for is one that works good enough we don't know right off it's the wrong prop for that plane/engine/fuel/and flying site's altitude.

The best prop for YOUR model really winds up being (pun intended) one that lets the engine run in it's best rpm range while using the fuel you bought to pull the model you have in front of you (ARFs vary in weight off the assembly line and straightness and quality) at your flying field on the day you're out there flying. One size prop can be the answer, but it's not guaranteed.

So you look at the mfg recommendations for the engine. You'll see a huge range most times for the reasons already mentioned. Every one in that range has a good chance of working well on that engine. The best thing you can do is start with what you see working on your engine on your model at your field IF someone out there already has that combination and is flying it.

But all that aside, for a beginner's trainer, you want "more" prop diameter and "not so much" pitch.

The diameter gives thrust. It also, when "enough" for the engine, will actually help make that engine easier to adjust and smoother running in the air. 46s can run 10" diameters, but often are screamers with more sensitive needle settings with that dia. They'll turn 11" diameters just as easily when the pitch matches the airplane, and with a trainer that'd be both 5" and 6" pitches.

Once upon a time, glow .35s had the power to spin 10x6s and that's what you'd see on almost every one of them. Today's engines are a bit more powerful. A good .46 on 10% fuel pulling the average trainer would be underpropped with that.

Try either an 11x5 or 11x6 first off, and plan to try more than one brand. BTW, there is no magic prop manufacturer. Advice to the contrary is actually a clue that the advisor hasn't really tested different brands to know they all work very well when dia/pitch/blade area match up with the test plane/engine/field altitude/etc.

It's actually valuable experience for anyone who has just learned to fly by himself to start right in trying different props. It teaches them to better observe what the plane is doing etc.

Have fun, but that's the easiest thing to do in this hobby.