To adjust the logo width with the HTML5 email templates follow the directions below.
WARNING: Before you make changes to an existing template, it’s a good idea to make a copy of the text somewhere first. There is no concept of version control and no automated way to go back to a previous version.
1. Localize the Email Template you wish to edit
2. Search for the phrase “site_logo_email”
3. Arrow down until you see something similar to the following
<img height="auto" src="[% cms.media.get('name' : 'site_logo_email').view().resource_url() %]" style="border:0;display:block;outline:none;text-decoration:none;height:auto;width:100%;font-size:13px;" width="250">
4. Adjust the width as desired. The default is 250.
NOTE: While there is not technical limit for width, at some point it will be too big for a given screen.