Flymag header size

Hi all,

I’m currently working in a test environment with the Flymag theme. I want to make a customized header but the size that was suggested (1300 x 768) doesn’t fit. It fits when I’m at the ‘cropping’ page, no cropping required. But when I then publish, it only shows half.
I tried a lot of different sizes already and getting a bit desperate as I absolutely love this thema and want to stick with it.

Anyone who knows the solution or a proper size that works out well?

Thanks!