Facebook event cover size

1920 × 1005, which is roughly 1.91:1. Mobile crops it shorter and overlays the event name and date across the bottom, so keep that area clear.

Event covers get treated like a poster and that is usually a mistake, because Facebook already prints the event name, date and location underneath and, on mobile, partly over the image. Repeating all of that in the artwork produces a cluttered result where the two sets of text fight each other. The cover works better as a single strong image with at most a short line of type. The other consideration is that the cover is what appears in a shared link and in the feed when someone marks themselves as going, so it does most of its work at small size in a busy feed rather than on the event page itself.