RE: How does changing the prop length/pitch affect performance?
I just noticed your note about wanting to slow it down, & FLYBOY's suggestion of a 11-6 or 10-8. If you want to limit in-flight speed, use your throttle, that's what it is for. However, if you want to reduce the landing speed, you will get best results from a larger diameter, lower pitch prop (eg. 11-4).
Flyboy's suggestions will certainly reduce top speed, & the 11-6 will likely lower the landing speed, but the 10-8 will actually increase the landing speed, and both will absolutely kill take-off & climb performance.