With your form selected on the page you should go to 'edit design' in the property inspector. In here you should make sure that form is selected and adjust the form width setting to be like you need. If you cannot get the form to fit correctly within your page by adjusting the setting here please post back with a link to the page in question or with a copy of the page that has the form on it as well as the css and other resources that I would need to check on this locally.