I don’t think there’s a fixed aspect ratio or resolution size for community banners. But your instance might have resolution or file size limits for uploading photos. You can ask your instance admin about it.

Nobody knows!

Seriously, it’s been two months that I’ve been searching for this basic information and still nothing concrete (you’d think it’d be in the docs). FWIW, as close as I can figure it through experimentation, it’s roughly 4:1.15, e.g., 1920×552px or 800×230.