Staying away from the giants and jets the 48 inch oz servos are fine for most anything.

I have used them in a Robin Hood (99inch span, Zenoah 38 powered), a Senior Telemaster and a Ryan STAM ARF powered by an OS 4 stroke. All have worked fine with no gear stripping etc.

Years ago Bill Northrup flew something like a Gypsy Moth that must have been around 8 ft. span or so with a system that used pulse servos. They were a small geared coreless motor with a spring return. The output was less than our modern micros.

Years ago Don Brown flew in the old internats (and placed high) with a pulse system of his own design that used modified double geared mighty midget motors (if memory serves correctly). I have used them myself and again the modern micro servo has more output power.

I just saw a demo on the video Modelsport magazine where a brick was hooked to the output arm of a JR micro servo and it lifted the weight. Awfully impressive.

All of that is to indicate that the average servo is useable in most any airplane up to and including a 1.20 powered machine. Of course better precision and a safety factor is achieved when a larger servo is used.