Inserting images into email campaign header for first time....!

Hi everyone,

I'm creating a Squarespace automated email sequence for the first time and I'm struggling with an appropriate image size for the header section of the email.

Can anyone advise what the ideal px size should be to fill the header fully with one rectangular image please?

Have tried 1280 x 400 px and 2500 x 750 px but both images don't fill the header.

Any assistance greatly appreciated!

Many thanks, NMc

Did you ever find this out? Also having issues.

I'm afraid, I didn't - have gone with my logo for now. Best of luck.