a value of 75% should display at 75% of the settings inherited from the parent, or more precisely, already (via class, tag, etc).
a value of 12pt is, well, 12pt anywhere.
absent any other compelling selectors in play, parentNode.fontSize="75%"; should do what you want.
if you do have lots of selectors going around, you'll need to grab the appliedStyle then loop through kids applying a modification of that detected value.
let me know if you want a good xbrowser applied style code. Most of them stink; a lot of them ask "are you IE?" every single time they fire...