I haven't tested this but found the following information online. Hopefully it will be helpful. As a guess, I imagine you may need to add some css code to make it Mobile responsive. Good luck!
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(From 2017)
Make the banner to be 600px in width and about 125px in height. This proportion looks quite nice on CK emails. You can try 600px x 200px and see if you like that better but I think 600px x 125px works just fine.
Here's the link to the video training on How to Create An Email Banner in Less Than 5 minutes191
https://www.sophiaojha.com/blog/005-adding-banner-to-convertkit-emails211