Can someone explain how the pitch and length of the prop effect the speed of the plane/engine?

I flew my WM Super Sportster 40S today about a half dozen times, being this is my first plane after the trainer, it is FAST. I am running a 46FX on it, and this baby hums.

I want to slow it down a little, and not just with the throttle wise guys, but with the prop. I put an 11x4 on it, and it helped a little, but should I go with a larger prop with the same pitch? Any help is appreciated.