Facebook page cover size is very important in order to have your page display the same on both desktop and smartphone.
It is Facebook’s recommendation that your cover photo should be exactly 828 x 315 pixels.
These dimensions however causes the images to be cropped on a cellphone when viewing the page.
In order to solve this, you need to make your image a lot taller than the recommended 315 pixels It should actually be 465 pixels tall. This gives you exactly 75 pixels on both top and bottom that will be cropped on desktop.
I have tried a lot of different sizes and came to the conclusion that 825 x 465 worked the best. You may lose a pixel or two here and there but it will not be noticeable!
The best results will be achieved if you upload your cover from PC rather than cellphone. The compression when using a cellphone is much greater when uploading from cellphone and the image will not look as good.
Test Facebook page cover size here:
You can test this theory by viewing our Facebook Page on both desktop and smartphone.
I hope that this will be of some help to you.