How to resize an NPC's body part without breaking all of it's joint?

Note: This is mostly about building rather than scripting

Usually you can just resize the character in the same ratio, but if I want to resize a body part in whatever size I want, the joint will be broken and it will just falls apart when playing. So how do I prevent that, or how do I add a new joint for it?

EDIT: So I figured out how to do this. But here's an extra question, even if the body part size is changed, the rotating center of the Motor6D is still the same, so how do you customize that center?

Easy to do on R15 characters, but for R6 you need to make sure you don't break the joints when you resize the parts.