I've seen them anywhere from 4 x 4' to 8 x 8'. From what I've read they are either painted right onto the barn/house/shed or onto panels of 3/4" exterior grade plywood- primed, painted, and sometimes clear poly on top. Be aware- poly can yellow over time. Some are mounted directly to the building, some slide into brackets so they can be changed. Making one is on my "someday" list. You can research the process in more detail on the net, including pricing. A few years ago I saw prices of $200 on up, sometimes including mounting, sometimes not. They are Heavy! There's a Barn Trail in Neversink, NY that's on my bucket list.