When creating a banner for an email marketing campaign using one of our email templates, we recommend designing the banner to be 600 pixels wide. If you are concerned about an email being mobile friendly, using a width of 300 pixels is recommended.
If you are not using one of our newsletter templates (working with the blank template), you would not have to worry about a specific banner size; however, you would not want to have a banner that is too wide because not all recipients will be using high screen resolutions. Always consider mobile devices!
Another factor to consider is the viewable area of the message. Sometimes people read messages in a preview instead of opening the email immediately, and a narrow banner will look better in a preview window.