Hi @SomeOne, thanks for the comments.
My print shop (specialty Fine Art printers) said they just print 300 DPI, and they can print 44 inch or 60 inch width. And they can print any length you want, it’s a rolling print thing like they do these days. (And you need to leave a border, less than an inch say). So my 12000 x 22000 image would be 40 inches x 73 inches, which is big certainly but reasonably routine I think for digital gallery-quality art.
That’s the plan anyway !
Ciao.