A larger diameter prop will not allow the engine to rev up as much (top rpm is reduced). The pitch will determine how fast/far you can go.

The pitch is how far the propeller will travel in one revolution - a 6 pitch will THEORETICALLY travel 6" in one revolution. In the real world it is a little less.

As for prop size, I suggest starting with what is recommended in the owner's manual and then adjust the diameter/pitch based on what you want/need the plane to do.