myMovie._height on dynamic content in that movie

I have a movie that I am loading text into. The textbox is set so the text flows downward. In an ifStatement further down, I am trying to see how tall that movie is(depending on how much text goes into it) to see if I should attach a scrollbar for the movie. If I trace the hight, it tell me that the movie is 200 high, which is the size that I created the textfield in that movie. But when the text is in it, it is actually much taller.

How can I get an accurate reading of how tall it is, after the text is loaded into it?

Thanks a lot!