Creating a Scale/Transform controller

Hello, I'm trying to model/rig a character that can grow/transform limbs and trying to find an easy way to repeatedly animate the effect. Is there a way to set up a custom controller to either scale just one portion of the model or would it be easier to set up shape keys for each transformation?

A good example of the effect I'm trying to replicate would be something similar to Luffy's Gear 3/4 or Generator Rex.