I have now managed to create a 16-bit png image that is 65,536 x 32,768 and 1.5 GB. I thought about stretching the grayscale out with auto levels, but it seems that for the displacement map or bump map purposes of this image, the way it is is better.
The image is available for download at https://www.moonwards.com/project.html#assets.
A set of 32 tiles taken from that image is about to go up too. But, i have a question about the results, which i'm about to ask.