That was exactly what I was thinking. I use Sketchup Pro for my 3D modeling, every new model starts with a default scale two dimensional abstract person who is transparent from all angles except for one face. I have taken to keeping the scale person in my models for AR to act as a point of reference for scale such as in this model. I’ll typically place the scale figure near the model’s point of origin for scaling. When I want my model to be approximately one to one scale, I’ll scale it up until the scale person’s eye height is about the same as mine (approximately 1.70 metres in my case). Another guide you can use is scaling their eye height up to where the horizon line would be (the perspective vanishing point). I then rotate the model to make the scale person disappear. This Instagram video demonstrates the technique too. I’ve found this technique to work for my purposes.
Another technique is to subtle mark somewhere in your model which is at eye height.