Actually, that's wrong again. The reason full scale helos can't go faster than about 300mph in level flight is that the leading blade is approaching high mach numbers at that point, and compressablity issues start to show up in bad ways. And if you pick up a shock wave in the rotor blades, your helo is now an elevator minus the cables.

But in general, a low pitch large diameter prop, or a prop with large chord blades will act as a brake and slow the plane down. Also, since the theoretical airspeed of a prop can be calculated to be pitch*rpm, if you lower the pitch, the plane will fly slower. In high speed flight, the difference isn't that noticeable, but in a landing approach, a few mph slower at idle can make it easier to land. Espeically since you have a definate min RPM most engines will idle at. Add on to that fact that our engines idle better with longer, low pitch props or props with heavier blades due to the increased rotational intertia, and the overall effect all adds up to make it easier to slow down a plane with a larger, lower pitch prop.