Facebook cover photo size
Facebook cover photos look like banners on top of your Facebook page and should be 820 pixels long by 312 pixels tall on desktops and 640 pixels by 360 pixels tall on smartphones. This size works for Facebook Pages, Facebook Groups and Event Covers.
Also you can use 1920 px x 1080 px, this displays perfectly on mobile and, if you center your key messages, displays very well on desktop.
Facebook cover photo format
FB Cover photo should be JPG or PNG file that weighs less than 100 kilobytes, for images with a logo or text, it might be best as a PNG file.
Why your Facebook cover photo matters
Facebook photo cover is a small but significant tool when it comes to promoting your business, services, or your personal brand. It’s the first thing seen by anybody who visits your Facebook page.