Your default email width is set globally for all emails in your content settings and cannot be changed in the email editor, changing the width of a single email requires editing the custom HTML version of the email template. This is only recommended if you have a basic understanding of HTML.
Note that if the width of an image or a table in an email is wider than the email body, the email body will automatically resize to fit the width of the image. To change this, find where the width is defined in the code of the image and replace it with a smaller pixel value.
To create a coded email template and set the width manually: