Dynamic text field height

i have data being read from an xml file. flash then displays one of the xml attributes in a dynamic text field. my dynamic text file is set to multiline.

how do i create a background for this text that adjusts to the height of the text that is displayed? for example, i must draw an area for the dynamic text field to display up to four lines of data. i would like to create a background below this text that adjusts to how many lines of text there are. how can this be done?

any help is much appreciated. thanks in advance!