The best rule is to not put your thumb anywhere near the prop, especially when it's spinning

Generally speaking: Increasing diameter increases thrust. Increasing pitch increases speed.

This whole electric power system thing is a balancing act. It's the same with glow, but with 50+ years of experience, it's not nearly as experimental in nature. An electric motor and battery can turn a very narrow range of propellers while still achieving peak performance. Get too much pitch or too much diameter, and you burn something out. So, be careful.

Electric corollary: Increasing diameter or pitch increases Amps.

To keep the Amps from getting out of hand, when you increase one, you need to decrease the other. Add an inch of pitch? Take out an inch of diameter, UNLESS you have a Whattmeter and/or know that the increase won't exceed any rated capacities.