Shorter prop with more pitch= a faster plane with less pull or torque. Longer prop lower pitch=less speed but more torque. I like to go into a stunt slow and have the pull to do a loop as big as the sky with as much vertical pull as I can get from an engine/plane combo.
If you are looking for speed you wouldn't like my planes, well, maybe you would. I run a YS 1.10 in a 60 size Hots but to slow it down I use a 16X6 on it. I have a 60 size Kaos with an OS .91 FS and it uses a 16X 6 or 4. I can slow my planes down a lot for landings with these props on them and my take offs are really short, the Hots doesn't need to move forward at all!!
I didn't ask anyone else what they were using, I did what John does and spent the time testing until I found my magic prop size, that's the one that will fly the plane the way I want and keep my engines in there working RPM range. Testing is fun and a big eye opener. You can't have too many props!! Nuff Said!!