A long time ago, I ran 6x 21.5 inch full-HD monitors, in a 3 by 2 layout.
This gave me a resolution of 5760×2160. A problem with this resolution is that you could not find any sensible background image, at least back then.
This led me to create this script, which creates an image of appropriate size and fills it with some nice colors based on your input.
These are the results:
And the script:
Generate your own background