How can I limit the writable space in hyperref's TextFields?

How can I limit the writable space in hyperref's TextFields? I'm creating a pdf form using hyperref. After it has been filled-out, the form will be printed. I'm looking for a way to prevent the user from entering more characters into a TextField than will be seen on the print-out. The maxlen option is not really what I'm looking for - it's more about space than number of characters. Incidentally, I do get some of the desired results when using the multiline=true option, at least in Adobe Reader 9 (which will be used): entered text breaks at the visual end of the TextField. Sadly, the number of rows increases instead. So I'm stuck with the same problem - only now it's vertically, not horizontally. Any ideas? Thanks!