Help with setting text boundary with action script!

Hello, friends. I have problem with set the boundary for the textfield.

I have movieclip called screen_mc and inside I have movieclip called text_mc which contains a dynamic textfield. And I have two buttons for increasing and decreasing the text movieclip’s size. But I would like to have it work like no matter how many words I typed or how large I press the increase button to increase it, the text is within the boundary of screen movieclip.

Does any one has some idea how to make it? Thanks a lot if you have any idea or suggestion. And wait for your reply!