@mntmn Another interesting smaller form factor option is Mini-STX: https://en.wikipedia.org/wiki/Mini-STX
There aren't too many cases or motherboards out there, but it is a standard and they do exist, and they would also make for some nice tiny computers. That said, I think that 1L desktops are quite a bit more common than mini-STX desktops.
@leimon @mntmn also there is micro stx but it's a lot less common than mini stx
https://www.asrockind.com/en-gb/micro-stx