Ray: Both have an effect on RPM and power. Usually, go down in either diameter OR pitch will cause an increase in RPM.

Consider this, for either a two stroke or a four stroke:

The prop is like the transmission in your car. If you want to climb a steep hill, you can change to a lower gear. Your car will go slower or your RPM will increase, but you will pull much better.

Now, shift up to a higher gear. Your car will go faster, but your engine RPM will decrease, or you will go faster with the same RPM as you had in a lower gear, but you will have less "power" to the wheels (the engine is generating just as much "power" but the demand because of the gearing, will cause the car to accellerate at a slower rate.

Same with props. Increase the pitch and you will go faster. Decrease the pitch, you will go slower for the same RPM, but the engine could over-rev too.

This is probably beyond the scope of this topic, but maybe it will help.. and maybe you know this already, but here goes:

Pitch is how much or how far the prop will move forward for each rpm. A 10 picth, for instance will move forward 10 inches for every rotation. Multiply that times the engine RPM (Revolotions per MINUTE), say 10,000 RPM and you have 70,000 revs per minute or 70,000/60 for 1166.666 inches per second or 97.xx feet per second. 60 miles per hour is 88 feet per second, so your prop, at full RPM and in the air AND under ideal conditions, will travel somewhere over 60 miles per hour.

Lower the pitch, and that speed will decrease.

That's speed. Equate that to power as you would a car transmission and you can see how this relationship works. Note, this is under ideal conditions.. no drag, perfect conditions, which do not exist.

Hope this helps.

CGr.