I seem to encounter a lot of problems with getting models to the correct size when turning on AR.
Here is a link for some simple boxes that you can change the size of:
The size of boxes is in mm.
It seems that it loads all models at the same size, to begin with.
I have tested with a lot of different models now, and the scaling factor is not the same.
I also have problems loading large models (under 20 MB).