I think he only wants to make the weapon bigger, this is impossible without custom models, especially if you for example want to size up arthas frostmourne in the actual arthas model.
The simplest way to do it is by importing many attatchment models of the same model with different sizes and attatch one of them to a weaponless model(which you can request at this site).
Whenever the health thing increases, exchange the attatchment for the slightly bigger one.
If you've played pimp my peon you should know that it uses a lot of attatchment models, but they increase the units size in the trigger editor.