It looks like Demon is doing the best it can with the resolution you've got, but 720 is as small as it can go on the Y axis. :( If F11 is working happily, my recommendation would be to continue using that; it is unlikely Demon will support window sizes smaller than Nx720 in the near future.
Demon doesn't have its own Discord, but we do have a channel on the roguelikes Discord. :D This link should be a valid invite for it: https://discord.com/invite/HVBmg7