IMHO, the forumla of pitch x diameter = "loading" isn't really accurate, while you might use it as a ballpark, you should also keep in mind that it is one rough approximation. (my personal experience tells me that a 9x6 and an 11x5 aren't even close to the same load, but ymmv)

Another forumla for a "loading" is the 2 x diamter + pitch, so a 10x6 and an 11x4 would have the same loading of 26.

I've heard of both of those forumlas, and a few others as well, and none of them are accurate enough to tell you what prop to use, you have to bolt one on and try it, nothing else really works.

Some of the factors that make the various "load formulas" not work well in the real world include:
- Engine torque curves are not linear
- prop efficiency isn't 100%
- prop efficiency actually changes with RPM, espeically at high RPM
- different brands of props in the same size will load the engine differently and fly differently (try a MAS and an APC in the same size some day)
- different prop materials will act differently, wood vs plastic for example.

The points everyone made about diameter vs pitch is right on in concept, lower pitch is good for climb and acceleration, high pitch is good for straight line speed Assuming the same RPM! Sometimes you can actually go faster by lowering the pitch and allowing the engine to run at a higher RPM for example.

Prop selection also depends a lot on the design of the engine. Some engines are "long stroke" designs, with a small bore and long piston stroke. These engines prefer to turn larger props at lower RPMs. Other engines are "overbore", with a large bore and short stoke. These engines want to rev much higher and turn a smaller prop for better performance. Most sport engines are roughly "square", with the bore and stroke about the same. And there are other factors as well, such as port timing and carb design. So, it is quite possible that two different engines will do different things with the same pair of props. I've seen this myself, the Mag .15xl will turn a MAS 7x4 better than an OS .15LA. But if you put an 8x3 on both, the OS seems to have an edge or at least is much more equal.

Generally, when I have a new plane, I'll pick up a prop or two in a few different sizes from different companies and try them to see what happens. I don't try everything out there, just a couple based on past experience or what other guys are using. The more I care about getting every last bit out of the plane, the more props I am going to try. After a little while, you wind up with a box of different size props, which makes experimenting later on easier. I've also been known to borrow or trade props at the field to see how something works.