The general rule of thumb is that if you go up one in pitch, go down one in length. If you go up one in length, go down one in pitch.

What is the pitch? Let's say I have an 11x4 prop. The pitch is 4...That means that if the propellor was 100% efficient it would go forward exactly 4 inches for each revolution. Obviously our airplane engines and props are not 100% efficient, which is why prop choice is such a big deal.

Think of a longer flatter prop as a lower gear in a car, first gear.

Think of a short curved prop (high pitch) as a very high gear in a car, 5th gear.

If you want an airplane that accelerates quickly, holds a constant speed, and gives you the most thrust you want to be in first gear. This means that you will reach a certain speed and just can't go any faster. Your acceleration will be great! For a .46 size engine this would be something like a 12.25x3.75 prop.

Now let's say all you care about is top end speed. You want to be in 5th gear of course...Your acceleration will be slow, you will lose speed in turns, gain speed with the nose down, and not be efficient at all when you're at slow speeds. For a .46 this would be something like a 9x8 prop.

Keep your engine happy and somewhere within the recommended prop ranges. If you ever experience overheating or have a hard time getting the high end/low end set, you've probably gone too far.