I don't know that you need to lock the healthbar size. Suppose it looked something like this:
********** 4So I'm at 100% health, with 4 health tanks. In other words, 500% health. I take 80% of my base health in damage, which leaves me at:
********** 4So I'm hurt, and I know I'm very close to losing a health tank. Then I take 40% damage:
********** 3I've lost a health tank permanently (until I gather vitality zones and use them). This process would continue until I lose my last tank at which point the # of health tanks displayed would probably just vanish (instead of showing 0) and the actual look of the health bar itself should probably change slightly to give additional visual warning you have no fall-back once that one is gone. Maybe give it a red border or make it thicker.
Edit: Bah, doesn't like ASCII characters
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