I also e-mailed Dan Ebberts who gave me this advice:
"
Doing it based on the number of characters should be pretty easy, you just need to define how many characters corresponds to 100% scale and ratio it from there. Something like this (I'm assuming you only want to scale the width):
T = thisComp.layer("some text").text.sourceText;
charsAt100 = 20;
w = T.length/charsAt100*100;
[w,value[1]]
The charsAt100 variable defines how many characters equate to 100% width.
Doing based on the actual size of the text is trickier. You have to use sampleImage() to find the extents of the text. I think somebody posted an expression to do this at AE Enhancers.
Dan
"
Thanks again Dan