Hi Bing,
Have you set the textfield to be multi-line, so that text will wrap within the field?
Also, have you saved your PDF as a dynamic PDF Form?
To make a field growable (expand based on user input), the field's parent needs to have a flowed layout, which your XML shows. The field needs to have a minH attribute, and the PDF also needs to be saved as dynamic.
Based on your XML above, I would guess your PDF form was saved out as a static PDF form from Designer.
Regards,
KJ