Yep, that's why the scale guys use display props .

The Alpha trainer uses the Evolution engine system which comes with a 3-bladed prop with some huge flat blades. The prop does do a really good job of slowing the plane down, while still providing a reasonable amount of thrust. But when guys replace that prop with a normal 2 bladed prop, they are requently surprised at just how much more thrust and speed they get out of that engine.

As Aaron said, multi-blade props are good for quieting things down and for ground clearence. (WW2 fighers used them almost purely because of ground clearance).

There are several "prop size forumlas" like the one that Aaron gave. The fact that there are so many makes it pretty clear they are all rather rough guides. You can (and maybe should, if you aren't happy with the planes performance) experiment with a fairly wide variety of prop sizes to find the right balance of thrust, speed, noise, etc. Also, differnet brand props can act very differently in the same size.