Facebook Cover Photo Size:
Optimize your cover photo for the right dimensions:
851 pixels wide and 315 pixels tall for desktop,
640 pixels wide by 360 pixels tall for mobile.
If your uploaded image is smaller than these dimensions, Facebook will stretch it to fit — making it look blurry.
So. if you upload a facebook cover photo, it should be suitable for both desktop or mobile.
the perfect size should be: 851 pixel wide and 360 pixels tall.
The grid picture above, you can see the red area. Place all your important information inside this red area.
then facebook cover will show in this way:
desktop or notebook version:
in mobile version:
no wording will be cutted by facebook!!
Don’t worry about the old ‘20% text’ rule, but still try to stay visual.
Facebook removed any reference to the 20% rule on text in cover photos … but that doesn’t mean you should go wild with using text in your cover photo. The previous rule said that only 20% of your cover photo could be text. Personally, I thought that was way too restrictive for marketers, but the sentiment behind the rule was a good one.
Save your picture in png format
If you want to keep your picture in high resolution, please save your picture in png format.
Details please read : How to make sure your facebook photos display in the highest possible quality?