First, I am sorry about replying late. Most of the team is on winter vacation, but all should be back to normal starting again in January.
Yours is a very good question. We don't have a set size for the header image, but 1600x400 should be good. We scale the image to fit the full width of the page, depending on the screen size of those who see the page.
In any case, you can try out several designs and sizes and see which works best.
I hope this helps. 👍️