There are two possible approaches:
1. You can force the Enter key to function the same as Tab: see "WD: How to
Code ENTER Key to Move to Next Field in Form" at
http://support.microsoft.com?kbid=211219
2. You can put the form field in a table cell with row height set to an
exact amount. This doesn't prevent users from pressing Enter, but most of
them will get a clue when their text starts to disappear.