Hello. The form is not customizable at this point. It uses the standard form info for Authorize.net. In the future we intend on adding form customization capabilities, but at this point, that is not there.
Just in the form of updating this thread... DT Register does offer custom form fields as of version 2.1.0. You can make text box, dropdown select boxes, radio buttons, checkboxes and textareas. Each can be customized in size, values, default selected values, etc.