It largely depends on how you construct the rig. If it's just a simple joints+skin setup, then simply scaling all of the joints at the same time will do the trick. If you group the joints and geometry together under a control curve or other and you want to scale the rig through that, then go to the geometry's Transform Attributes and uncheck "Inherits Transform". Then you can scale the control curve and the rig will scale with it.
If you're talking about creating a custom attribute on some object to control the scale of the joints/rig, then you'll want to use the Connection Editor and connect the scale attributes of the joints/control curve to that attribute (and preferably make sure that when you first create the attribute, its default value is 1).
For more complicated things like stretchy IK systems or anything requiring the length or distance between objects, you'll have to feed the scale of the control into the process in order to account for different sizes.