HELP!!! (please)

I need help. I need to make a multiline text field that does not scroll, it just stops letting you type when you have filled the text field up!

I could set the text field to limit chars, but that doesn't solve my problem, because the users will be using the return carage, and that would still allow them to type past the bottom of the text field. I just need to limit them to 20 rows, and 40 coloms of text. is there an actionscript way?

I am using flash 4, but would be willing to use 5, or MX if I need to to make this work.

Any one have any idea? Thanks!