If you upload a Facebook cover photo at 820 x 312 pixels, the center 555 x 312 pixels will be viewable on mobile. Alternately, you may also design your desktop cover image at a 16:9 ratio – in other words, 820 x 461 pixels (technically, 461.25 pixels).
https://sharethis.com/best-practices/2018/09/perfect-facebook-cover-photo-size/

