I got mine from Skiwhiz that has owned a few, and it is the 7" model.
He told me the larger coil will hit on really small stuff about an inch or two deeper than the small ones he has owned in air testing.
I have never held a small coil model, but I can't believe the separation can be that much better than the 7".
In my experience, that ability with the 7" is in the realm of surgical precision.
Trashy sites are no problem, sizing ability just by swinging over a target is phenomenal, and the sense on mine has been turned up to just below max so my depth is pretty close to 8" in the ground or even a little deeper.