In the Look & feel step > General, you can set up a width for your iframe. The width you choose can have an impact on how your banner and footer will display.
So how do you know what width you should give your iframe? This information comes from the website you are embedding your iframe on. To know the exact value, you'll need to check the source code of the webpage and spot the width of the container (div) where your iframe will be published. Please contact your webmaster if this sounds unfamiliar to you.
Nevertheless, Qualifio iframes are 100% responsive: a 1200px iframe in an 810px container will be resized to 810px (note that in this case, the banner of your campaign will replace your background image). And an 810px iframe in a 1200px container will be automatically centred in the container.