You can use the form_wrap class to change the width of the form background color. So for example, you could add the following to your theme.scss file to change all the form background widths to 610px:

.form_wrap { width: 610px; }

If you only want the change to appear on that particular page, insert this style only into the template of that page.

To change the width of the form itself, add the following to your theme.scss file (or to the page template, depending on where you want this change):

.one_column .form { width: 600px; }