The space saver that fits in the cavity under the luggage area is 17 inches. You won't get a anything bigger to fit in there, if that's where you were intending to put the space saver.
The right sized space saver can be fitted by removing the polystyrene packer and placing the sub woofer inside the space saver rim.
Be aware that the VXR has a different stud pattern to the other models. The space saver that fits the LT, RS and RSV won't fit onto the VXR.
I believe the VXR stud pattern is the same as the VE/VF, so a space saver that fits one of them might be what you need.
This thread talks about the stud pattern and shows the space saver tyre in the cavity under the luggage area.
https://forums.justcommodores.com.au/threads/punctures.287616/page-8