In our club, we say go w/ the biggest engine reccomended, 'cause the power will be there if you need it . If you don't need or want it, that's what the carb. is for, you can always throttle back. And a bigger engine can help balance the plane, because you don't want it tail heavy! Prop selection is pretty well known for a certain eng. size (there are charts + tables) or you can ask anyone here, just tell us what plane you've got, and eng. and someone will be glad to help out. Happy Landings...