Use a text form field--you can even set the maximum length. More precise
instructions would depend on what version of Word you're using. In Word
2007, you'd use a legacy text form field, which is available in the
Developer tab (Word Options, Popular, Show Developer Tab). Use the Legacy
Tools button to choose Text Form Field. This presumes that you're going to
use this as a protected form in which the only place for text entry would be
the form fields. If you're using Word 2003 or earlier, you'll find the
controls (not called legacy) in the Forms toolbar.
For less stringent control, you could use a content control (plain text or
rich text).... assuming you're using Word 2007 or Word 2010 beta.
--
Herb Tyson MS MVP
Author of the Word 2007 Bible
Blog:
http://word2007bible.herbtyson.com
Web:
http://www.herbtyson.com