Mike East's post covers the range.

He kept it simple and it is.

There are two things to keep in mind along with that advice. If you fly a very fast airplane or very heavy airplane (they wind up going fast) look for servos on the strong side of Mike's ranges. If you fly an airplane with bigger than normal surfaces (the movable parts are often called surfaces) go for the stronger servos. 3D airplanes have "larger than normal" surfaces, btw.