Form Width Question

I am including the contact form in a sidebar. The width of the sidebar changes depending on the orientation and device. Must the forms have a fixed width or can I set something so that it adjusts automatically depending on the width of the sidebar? Otherwise when the form is wider than the sidebar the overflow extends outside of the web page. That’s a problem. Tanks for any insights.

http://wordpress.org/extend/plugins/si-contact-form/