The header background image does not have a definitive fixed size.
It is a background image that fills the header area.
The size – and the width/height ratio – of the header area will vary depending on the size of the screen, so the background image will cover that area without stretching the image. This does mean that sometimes either the top/bottom or the sides may be “cut off”.
If it didn’t “cut off”, the background image would either have to strech to cover the height or the width or to avoid stretching it would be cut short and you would have empty space at the top/bottom or the sides.
I hope this helps to explain how the header background image works.