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.
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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!