Health Bar in Platformer

So I have been trying to create a health bar for a platformer and the only thing stopping me is how to scale an image in the gui builder. I would use getExtent(); but that returns a string with width and height. What I want to do is simply reduce it's width as a % of health. That's not hard to do but I need a way of isolating the width from the height.

I can't use a regular object and have set width and height because I need the bar to move with the camera