MailChimp Templates are designed to look best in the preview pane across different email programs and aren't really designed to stretch beyond or shrink below the 600px width that we've set.
The underlying Template code for width cannot be altered automatically or through any setting in the Campaign Builder. If it's necessary to create an email wider than 600px we recommend providing your own code or hiring a designer to create the code for you.
However, if you'd like to use one of our Templates rather than coding your own, there is a work around you can use to force your email to stretch beyond the recommended 600px width when working in our Campaign Builder. This will not change the underlying Template code. If you want your email to be more than 600px wide, edit a text content block in your campaign.
Then, upload an image to that text area that is more than 600 pixels wide.
After adding the header image you'll then need to insert a thin "shim" (transparent) graphic that also stretches the body and other sections of your template to the same width. If you haven't applied any padding to the body area, your shim graphic would be the same size as your header graphic. If you have added padding to the body area, make sure to subtract that padding from the width of your shim graphic.
Note: You don't have to do this for the footer as it will grow along with the body.
After stretching the campaign to a width larger than 600px we recommend testing the campaign in as many clients as possible to ensure that it displays as intended.
This feature is available to users with Author level or higher permissions. Take a look at this article to learn more about the different access levels for a MailChimp account.