I don't know too much about this, but I believe the simplest way to do this is to add a reference to each of your character files in a new scene. Then you are able to scale each character and furthermore, you can change your original files and the animation file will update itself with the changes.
Make sure your character files are nice and cleaned up and then I believe there's a file > add reference or something like that.

Maybe someone can elaborate on this or correct me, but hopefully that gives you something to go on.