the best size of header image : heigth and widht

good day dear wordpress-experts and users

first of all – i really love this great communty it rocks!!!

the question i have at the moment; which is the perfect ratio of header image (height & width)

howdy dear wordpress-experts

i am a bit confused: i need to know the possible sizes of the header image?

this is the question of the day for me!

well to begin with the beginning.

well i am fairly new to wp
i run version 4 and i am very very happy with it.
i run theme responsive (pro) and i am also glad with it.

can you help me!? that would be perfect