You could insert your form field into a table with a fixed width and hieght
and limit the number of characters your form field can accept. It really
depends upon what's filled into the field. If it's a fixed amount of space
(say 10 to 20 characters) then you could do it as I've described above. If
your users will be entering a lot of characters, you'll want the field to
expand to fit the text, otherwise you'll upset your users because not all
thier words will show up.
I am presuming of course that you're locking your document so only the form
fields are accessible.