Navigate to Email
From your HubSpot Dashboard, navigate to Content > Email.
Manage Email Settings
Once in the Email tool, on the left-hand side, click on the link to Manage Settings.
Email Size is a very small section of your email settings and lives just below the Email Footer Information section. Scroll down to see it.
Setting email size
You can see above that the Email Body Width is set to 600 and the Email Body Padding is set to 30 (for a total width of 660px). The Email Body is where you content will actually live - any text or images you insert will live there. The Email Body Padding is the space within the Email Body between the text and the border. It's a good idea to have some padding so that your text or images don't hit the edge of the email but have some space to breathe on the sides. Aesthetically, this can be a good choice to make.
If you want to edit these options, simply click into the Email Body Width or Email Body Padding text fields, enter the number you'd like for each, and click Save changes.
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 but it can be done. Take a look at this article that takes you through the steps of adjusting the body width of a particular email.