The Facebook Event Cover size (1200×628px)
A Facebook event cover uses 1200x628 pixels at 1.91:1, essentially the same wide banner shape as a link preview. This image is the first thing invitees see, and it also travels: when people share or the event surfaces in feeds, that cover becomes the thumbnail, so it does double duty as both header and promotional graphic.
Because the cover appears at several sizes, from a large banner on the event page to a small card in the feed, the key details need to survive scaling down. The date, time, and event name should be large enough to read as a thumbnail, not just at full size, and they should sit toward the center where cropping never reaches.
The most common problem is over-designing the edges. When Facebook reflows the cover into different placements, the left and right margins can be trimmed. Keep sponsor logos, ticket links, and any critical wayfinding text within a central safe band, and rely on a strong, uncluttered focal image so the event reads instantly even at a glance.