BTW, "if your plane is too fast, then...."

Did I mention that nothing in aeronautics is sound bite simple. An 11x6 will be faster than an 11x5, right. Actually, if the engine isn't powerful enough for the drag of the model you chose, an 11x6 might not be faster enough to look like it. And in fact, top speed isn't really the "fast" that matters. What matters is how fast you plane is in that prop test flight when it's coming in to land. Your engine is going to have an minimum rpm where it idles reliably. Most of us bring the engine to idle to land that model. And the speed at that time matters much, much more than when flat out.

Not simple is it.

A 3 blade will actually help some engines idle slower. (They certainly help some climb stronger and/or faster.) So will less pitch. And blade area helps sometimes. (something you get when you got 3 instead of 2 blades, and something you get with some makes of prop, and.....)