I don't know if this will help or not, but you could try it.
Instead of trying to resize the health bar, just create a new image and use that each time the health changes.
So, you would create another sprite or something with another health bar, and set it to appear when the player gets hurt, for example.
You'd have to set up some code to make the proper health bar visible or invisible as the situation calls for it.
You'd have to use something like instance variables attached to the player's health to tell it which health bar to spawn.
Ex: if health=3 then set healthbar3 to visible and rest to invisible.
Anyway, I'm still new to C2, so I don't know if this helped or not, but hey.