Flash MX 2004.
text.autoExpand
Property; a Boolean value. For static text fields, a value of true causes the bounding width to expand to show all text. For dynamic or input text fields, a value of true causes the bounding width and height to expand to show all text.
The following example sets the autoExpand property to a value of true:
fl.getDocumentDOM().selection[0].autoExpand = true;