Engine depends on airplane design to extent that cubic inches come in to play, not all hp is the same the way a airplane demands power so a higher rpm engine to develop the same power as a lower rpm engine but using PSRU will work fine in some designs but be "wheezing" in other designs. Basically the draggier the airframe the more it is going to like cubic inches which is roughly equivalent to torque @ lower rpm and the cleaner the airframe the better it will accept an engine with a PSRU. It usually comes down to heat, If the higher rpm engine has to work really hard to make the hp then you have to get rid of the heat and that is limited by it's cooling design. So cleaner airframe usually means faster for a given amount of hp and faster means possibly the ability to supply more air for cooling. hp/displacement/torque/rpm/heat/airspeed/acceleration it is a balancing act.
Go through the Hovey pdf on propellers as an exercise just pick an engine a simple one and design one propeller. Much will become clearer.
Go through the Hovey pdf on propellers as an exercise just pick an engine a simple one and design one propeller. Much will become clearer.