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!
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 minutes176