Photo Sizes[ Edited ]
Your cover photo will be cropped to fit a 16:9 aspect ratio, needs to be smaller than 5MB. While a minimum size of 480 x 270 will actually cover (had to use sizes that small out of necessity) I find 1,080 x 608 pixels works best.
If you are using some common social media graphics templates or reusing images from other locale, the Facebook Event header image is 1920 x 1080 - also a 16:9 aspect ratio and works.
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