gus is pretty much right on. Moving to a bigger prop usually means less RPM, which might or might not fly "better" in the air. Also, a whole lot depends on the engine.

As an example, I was playing with props last weekend on my old Cap 21 that has an old OS .61FS in the nose. (that's the pre-surpass, 2nd generation 4-stroke).

I started with an APC 12x6. The prop tached at 9,600 on the ground and flew pretty well, but I had too much speed in horizontal lines and not enough pull in verticals.

I popped on a Zinger 13x5. The engine still tached at around 9,600. In the air, climb was a hair better, but the plane didn't have the performance I was looking for, it was too slow in level flight.

Switched to an APC 13x6. The engine still tached at 9,600. And I had more pull in the verticals than either previous prop, and the level flight speed was actually a bit less than the 12x6, but not much. Overall, it was the best prop so far. (The 12x6 was unloading more in the air apparently, leading to a slightly higher airspeed vs the 13x6, even though they tached the same on the ground).

Another guy at the field had an old 14x8 (we think, it was unlabeled). Put that on. Tached around 6,500 on the ground. In the air, the plane was ok, but climb and level speed were only so-so. Both the 12x6 and 13x6 were better.

So, I found out that this engine really doesn't want to run faster than 10k, no matter what prop is on there. The lack of RPM change from the 12x6 to 13x6 tells me the 12x6 is really too small for the engine. Not small enough to hurt anything, but I can do better. The 14x8 was too big. If I wanted to fo fast, a 12x8 wouldn't be out of the question, but that's not what I'm looking for. I gotta get my hands on a 14x6 to see how that runs. Based on the 13x5, I'm not going to bother with a 14x5.