I want to work with UV maps that are not square. For one long, thin structure I want a map 8k x 1k. However if I save a normal looking UV layout to an image that size (with Export UV layout) then the resulting image looks like the UVs have been scaled in the Y axis down one eighth size. So is doesn’t fill the space. To get it to work I have to first scale all my UVs up 8 time in Y, then export the UVs to an 8k x 1k image, and the image looks right. Surely there’s an easier way.
It depends on the image you have loaded. If you use generated images, you can set that size to 8k x 1k and get the right dimensions that way (and also the correct preview in blender) or else you can load an image with that dimensions and it will work, too.
Edit: You set the size of the image on the left panel in the image space under “Image”