Board Thread:Support Requests - Designing Your Wiki/@comment-35289850-20200817213219/@comment-45117243-20200818004655

I think it's pretty logical. The bg image is shown at the sides, so 2000px leavs 400px on each side. This is a reasonable requirment. The current image you have is wider than 2000px and yet you asked to strecth it more...

I think you won't find a fixed size or proportion, as it differs between devices.