That’s odd, i use impact and the templates seem to be spot on, never had to re size at all, I have to take exact measurements before print on panel floods to make sure they fit as they always seem to be a few mil out and the panel shapes are never correct but apart from that, when I open the templates in CDRX5 they are always full scale.
Here is something I would try if scaling up.
Measure a small component on the vehicle such as the window width and then scale your image using that measurement in your image editing software. easy enough to do if you use the grid background in options you should get roughly the right size.
I have a red magnetic strip 1000mm long that I put on the side of vans if I have to take a picture to use as a template, I take the picture square on and then scale up using the magnetic strip as a scale guide.
Hope this helps a little
BigMo