The width of a HubSpot email header image depends on the layout, but a good rule of thumb is to use 600 pixels as the widest image.
The recommended HubSpot email header size should match the width of your email body width setting. You can check your email body width value by going to HubSpot settings > marketing > email.
By default the body width is set to 600px wide. So your email header banner should be 600px wide and any height. But a good general size is 600px W by 200px H. If you have any text in your header image, make sure to experiment viewing your email on mobile and desktop because your image width will be scaled down and your text may not be readable.
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