When you create the form in form builder, you have the option to style the form however you like including the width of the entire form... simply set the form's width to the correct width. Once you choose your style... just hit the button to customize it and change the width, and maybe even number of columns down from 2 to 1 if your form with needs to be really narrow. Then you could save the customized version of your style as a preset to reuse anywhere in your site and automatically have the right size forms.
Best regards,
Brian