What is the best width for a banner image in Enhanced letterhead header and footers?

My org is ready to make the switch from Classic Email Templates to Lightning Email templates. This involved recreating our letterheads. My marketing manager doesn't want to waste a bunch of time testing, so...

• What is the width limit on the header and footer spaces in Enhanced Letterheads?
• What is the best practice on the width (if different from above)?

Thanks in advance for your assistance!