We do not have officially recommended dimensions for header images, as these banners are rendered differently based on the size of the device's screen.
We find that 1280 x 300 pixels will work for most devices, but the positioning and scale may change depending on the device's screen width and resolution.
Generally, header images will be horizontally centered and top-aligned (vertically). Because the image will stretch according to screen size, we recommend using a graphic that does not contain text and is generated to stretch based on this alignment.