xml --> dynamic text box --> text box width

I am wondering about the following. I would like to have a mask reveal
over a dynamic text box, which gets fed by an xml document. I think the
problem that I am looking at here is that, I have to get the width of
the dynamic text box after the xml content was flowed into it. My
question is if the dynamic text box will have a width, dependent on the
content, not on how wide it is set on the stage? In oder for my reveals
to look right, I have to get the width, which will vary based on what
came from the xml file. Is this possible? I would like to avoid, having
to get a word count for each line.

Thanks for any help with this!