Email Header Image size: 600 x 200 pixels.

Most email templates are 600 pixels wide, a convention set by desktop preview panes and kept by every major sending tool. Mailchimp's image guidance asks for images between 600 and 1200 pixels wide in its legacy builder and 660 to 1320 in the newer one, and recommends a maximum file size of 1 MB. A header therefore spans the full column width, and 200 pixels tall, a 3:1 strip, is the common height: enough for a mark without pushing the first line of copy out of view.

Email clients do not scale images the way a browser does. Outlook renders at the pixel size written into the HTML attributes, and phones shrink the whole email to fit the screen, so type in a header ends up around half the size you designed it at. Set type large, keep it clear of the edges where padding and rounded corners can clip it, and export at 1200 by 400 with the HTML width set to 600 so it stays sharp on high-density screens.

Header images are often blocked until the reader chooses to load them, so the alt text has to carry the message on its own and the brand name should also appear as live text or in the sender line. A 4:5 social graphic does not belong in a header; it is portrait and pushes everything below the fold. Use it as a body image lower down instead, scaled to the 600-pixel column, where it renders at 600 by 750.