Fillable Form

Using Word 2003. I'm trying to make a fillable authorization form for our
website so volunteers can type in it at their own computer, save it and
attach to an e-mail and send in for an authorized signature. I have tried
making a form to put on the website, but the grey fillable area did not
expand when I tested it. The area only would only the number of characters
that Word allowed and did not "grow" to include more letters for an
explanation or even a name. Could you please help me so that the fillable
area expands? Is there anything else I should know when posting a form on
our church website so that no one can change the form's wording or content?
As always, thank you so much for your help!