We can make the email wider, however best practice is to keep the email width within 600 pixels. This is based on the prevalence of email clients that show email in preview panes that have limited space to display the email.
Here are some references to this best practice from others in the industry:
While screen widths have greatly increased on most new computers, email is best created for the lower common denominator and for email clients that still restrict the email size.
It is possible to create a template that has a more 'fluid' design for email. This would take up the full width of the available space. When you use a single column template this is easier to do. But to implement this, the banner image design would need to be split into smaller sections to accommodate a combination of HTML and images that will have a variable width. This requires a little extra design service, but it can be done.