Setting email size
From your marketing dashboard, navigate to Content > Content Settings. On the left, select Marketing > Email to access your email settings.
In the Size section, you can edit the Email body width and Email body padding. The email body width is the space for your email content such as text and images. The email body padding is the space between your email content and the outer edge of the email border.
To update your email width and padding, click the Email body width or Email body padding text fields and enter the number of pixels for the width.
Click Save at the bottom of the screen when you're done.
What if I want to change the body width of just one particular email?
If you'd like to change the body width of just a particular email, it requires some basic HTML knowledge to execute. Take a look at this article, which takes you through the steps of adjusting the body width of a particular email.