Facebook recommends 820px x 462px size for your Facebook cover videos, which we offer in the editor. It is important to note that other sizes can be used, as long as they follow the 16:9 size ratio.
When you create a Facebook cover video, and purchase the video quality download, you may notice that the download size is actually 1278px x 720px. This size is used so as to improve video quality when used on social media.
By comparison, a 820px x 462px will appear blurry as a cover video.
Don't worry, as the 1278px x 720px is perfectly usable as your Facebook cover video. Make sure the video length is greater then 20 seconds and you will face no issues while uploading.
Note: Keep important content and design elements near the center. Facebook crops the video from the edges to display it as a cover.