I am officially addicted to making Flash extensions. It seems to tickle some very nerdy part of my brain.
This one sets the size of the stage based on a selected element. That element can be pretty much anything on the stage (shape, symbol, text, etc).
Compatible with Flash MX 2008, Flash 8, and Flash CS3.
Download here.
You can do a similar thing in the Document Properties under ‘Match:’ and select the ‘content’ radio button. This is based on the whole timeline it seems, so might not always be what you want.
Thanks for the tip, Ranoka!