oUF_Hank is the only thing I could suggest. That is what I immediately thought of even thought it is not quite what you are looking for. It is a rather neat piece of work.
I don't think you can resize STUF that easily. But you can cheese it. Say perhaps every 25% your health drops your font would increase 2 points. Something along those lines.
You would have to confirm that I'm using its tag system correctly but this should work with STUF.
Example:
Text 1 >>> [custom_if_hp100: perchp]% <<<< set font to whatever and font size 10 set layer 1
Text 2 >>> [custom_if_hp75: perchp]% <<<<set font to whatever and font size 12 set layer 2
ETC
The catch is you would have to make a background for your font - which you can do with STUF. And then have each text box in its own layer. So that the newest size covers up the old one.
I hope that makes sense... its been a bit since I've used stuf.