Crop a Facebook cover photo
A Facebook Page cover is a wide strip, 851 x 315 pixels. This page opens on that size and lists the other Facebook sizes.
SnapCut crops and sizes the photo only. It does not check your face, expression or background.
Your photo never leaves this device. To check, open your browser's developer tools, choose Network, and crop a photo: no request carries it.
Facebook states the Page cover size on its Help Center, and SnapCut links to that page with the date it was checked. The cover is 851 x 315 pixels, a ratio close to 2.7 to 1, much wider than a normal photo, so most of a landscape picture is cut away from the top and bottom.
Facebook says it accepts covers of at least 400 x 150 pixels and that an sRGB JPEG under 100 KB loads fastest. SnapCut exports JPEG with a quality slider, so you can lower the quality until the file is small enough; the result panel shows the file size in kilobytes. A personal profile cover is different: it needs to be at least 720 pixels wide.
The other Facebook size in the picker is the Page profile picture, 320 x 320 pixels. SnapCut has no sizes for Facebook posts and stories, because no official source for them was found, and it will not list a size it cannot cite.
Pan the photo under the frame, zoom for a tighter view, and keep important faces and text away from the very edges. Check how the cover looks on your Page after you upload it.