OK, I think I understand now after looking at the PDF. You set the font size of the form fields to Auto, as opposed to some fixed size. This causes the font size to auto-adjust depending on how much text is in the field. So all you have to do is set it to closely match the text on the page.
I still don't understand about your desire to "get rid of the underscore on each side of the text in the field" though. Can you eleborate?