In PS12 I change the icon dimension to scale the background (6000X6000,4000X4000,...)
So far I haven't found the right way to adjust the position. If needed I modify the cad file.
I'll give that a try. I haven't been able to find a way either and I can adjust the original CAD file but I wanted to see if there is anything that can be done on the Plant Sim side. I'm going to create an IR on GTAC and I'll post my findings from them if any.
You can also edit the imported background. It is converted into a bitmap by Plant Simulation. So you can save it as such and adjust it in e.g. Paint. Afterwards you can import it back into Plant Simulation. The icon in question has the name "Background".