All files and scans on this website have correct measurements. However, it requires the right applications to be sure, when viewing or printing the files, the measurements are correct. I use Photoshop and when opening a file downloaded from this website, Photoshop detects the correct resolution and measurements automatically. My experience with standard photo viewing applications in Windows etc. is that it can do all kinds of weird things with measurements and resolution, resulting in incorrect sizing when printing it.
We do provide A4 and A3 versions of the boxes where applicable so you should be fine. If you then open it in a respectable photo editing application you should have no problem printing it at the right size. Also, each file includes a small scaling legend so when printed, you can check if the scaling is correct.
Good luck!