There would be no easy way to do this. You'd need to do some pretty fancy calculations, it would all depend on the size of the field, what font is used, the font size, whether it's been bolded, underlined, italicized, etc etc.
What are you trying to accomplish? Make it so that the user can type in the text field until it fills and then have it stop without scroll bars coming up to allow him to keep typing?
Chris
Adobe Enterprise Developer Support