The Facebook Group Cover size (1640×856px)
A Facebook Group cover sits at the very top of the group page and is the first thing members and prospective joiners see. The recommended upload size is 1640x856 pixels, a wide landscape shape that Facebook uses to fill the header band on desktop while still cropping down gracefully for the mobile app. Uploading at this exact width means Facebook applies the least amount of recompression, so logos and text along the banner stay legible.
The tricky part of a Group cover is that Facebook displays it differently across devices. On desktop the full 1640x856 is roughly visible, but on phones the top and bottom get trimmed and the group name overlays the lower-left corner. Because of that, you should keep your most important elements, such as a group title, tagline, or mascot, comfortably inside the central horizontal strip rather than near the top, bottom, or the lower-left where the name badge sits.
Designing at the native ratio also stops the awkward stretching that happens when you drag a mismatched photo into the cover slot. If your source image is a square or portrait shot, cropping it to this wide format first lets you choose exactly what stays in frame instead of letting Facebook guess. Resizing here runs 100% in your browser with no upload, it is free, and it preserves your chosen aspect ratio. Facebook periodically updates its layout, so verify the current spec before a big community launch.