What I really need to know is how do I get 10 inches of performance out of 6 inches of prop?
OK, sorry, I had too.
Generally speaking, you can go up in diameter until you reach a size of prop that is physically limiting the engine. i.e. your RPM's are so low that you are actually "loading" the engine too much, and you will do damage to the engine from overheating. As stated before, there are "rules of thumbs," but nothing beats real world results.
I would suggest you get a handful of APC props that you feel are within reason for your engine, and try them all. If you tell us the engine you are talking about, you can probably get some pretty specific suggestions to start out with.
Generally speaking, the pitch tells you how many inches that the prop will "screw" through the air with each rotation. Of course, these are ideal world numbers, and we don't live in an ideal world, we live in the real world. So efficiency really comes into play, and APC are great, affordable, efficient props for the average modeler, so therefore I recommended them. However, please realize these props are very sharp, and much less forgiving if you get your fingers into the prop disc than, say, a MAS prop would be. Of course they are both going to hurt, and send you to the hospital, but an APC is likely to send you missing some fingers all together.