Best Email header image size and resolution

Hi in my form: https://form.jotform.co/70125778033858 I have an automatic responder which has a header image.. what is the best size it should have to fit mostly hotmail and gmail?

I would like to apologize for any inconvenience, however, I'm not sure there is an ideal size for the header image in emails.

I can only advise you make test submissions and observe the size of the header image that would appear in your email. Update the size of the header image on every test submission to see which would best fit your requirements.

Thank you.

Do you have a personal recomendation? Or your team.

I used to build an email template, and my sizing is 600px minimum on the image width. Please try that :)