Someone answered in another news group:
Using instance names for text areas in MX, you can use the property
"autoSize".

"J-Dogg" <j-dogg@quitspammingme.net> wrote in message
news:SZqsb.18442$jA6.2267@bignews6.bellsouth.net.. .
> Hi everyone,
>
> How do I make a text field dynamically fit its contents?
>
> I have a movie clip instance, called "macromedia", which holds a dynamic
> text area, called "myText".
> The frame script in the mc says:
> myText = this._name;
>
> so when the movie is published, myText reads "macromedia".
>
> The problem is that I want the text area to fit the text, not just be one
> length and either crop the text if it's too much or be longer than the
text
> if it's too short.
>
> The purpose is to duplicate the mc based on the number of items in an
array,
> and fill the dynamic text area "myText" with contents of the array. Then
> each mc should be only as long as the sentence it contains, so it can
scroll
> to the end and stop within a masked area. Each sentence is between 50-100
> words, and will be supplied by the client the day before the CDs get
duped,
> so I'd like to make it as dynamic as possible.
>
> TIA
>
> Jeremy
>
>