Update the css for the form on line 36 of webassist/forms/fd_basic_default.css and change the width from 760px to auto.
Then you will have to add some media query css to adjust for small screens. I think this would do it:
@media only screen and (max-width : 767px) {
form.Basic_Default label.sublabel {
float: none;
text-align: center;
}
form.Basic_Default label.sublabel a {
display: block;
}
form.Basic_Default span.buttonFieldGroup {
float: none;
padding-left: 0;
}
}