The Email Newsletter Header size (600×200px)
600 pixels is the long-standing standard content width for HTML email, so a 600x200 header at 3:1 fits neatly at the top of most newsletter templates without horizontal scrolling or awkward resizing in email clients. This width is the safe default that renders consistently from Gmail and Outlook to Apple Mail.
Email is a uniquely constrained environment: many clients block images by default until the reader clicks to load them, so a header should reinforce your brand but never be the only place your key message lives. Keep the essential text as real HTML text where possible, and treat the header image as visual support rather than a graphic carrying critical information.
For sharpness on high-density displays it is common to export the image at double resolution, for example 1200x400, and display it at 600x200 so it stays crisp on retina screens. Keep the design simple, with your logo and a clean band of color or imagery, and make sure it reads well even when images are off and only your alt text shows.