Servos are generally required to control very small loads, in RC vehicles - throttle, steering, etc. They generally have limits of 0 to 180 degrees of travel.
Continuous rotation servos aren't really servos.
It sounds, though, like what you need is a stepper motor. They come in various sizes, from tiny little things to great big ones. I'd look into them before trying to find a continuous rotation "servo" that does essentially the same thing.