If you place each text form field inside a table cell whose dimensions are fixed, the layout of the form will be preserved. Most users will stop typing excessive text when they see that it disappears into the bottom margin of a table cell.
To actually prevent the Enter key from working in a form, you will have to make use of macros. See
http://support.microsoft.com/kb/211219/en-us/ (How to code the ENTER key to move between form fields in a protected form in Word).