If you're using custom HTML for your campaign layout, you can use a fixed width of 810px, or you can make it responsive.
The fixed width option is definitely easier, but a responsive layout will look great on mobile. To make the change, set the HTML option to "Fluid" at the bottom of the Customize page.
After that, there are just a few main things you'll need to add. First, add a meta tag to the top of your HTML:
<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0">
Now, all you need to do is set percentage based widths for your elements instead of pixel-based widths. You can also use CSS Media Queries to set different styles for different sized screens.
Here are some additional tutorials about responsive design in HTML. Note: these links are not created or maintained by Woobox.