tl;dr: As far as the MCU is concerned, the science in Ant-Man is both vague and contradictory enough that we can't really answer this question any better than what you already said.
The movie is rather vague about just how small Scott can get before it becomes problematic. I assume this is intentional, since any attempt to be specific would just invite criticism of the science :)
However, within the limited experience of the single movie thus far, it looks like the suit is already set up shrink Scott as small as he can safely get: about the size of an ant. We never see Scott use the suit to shrink to any other size -- nothing smaller or bigger than the one fixed size. The only time he tries to go smaller, he is forced to disable the "regulator" to make it happen.
Of course, it's possible (even likely) that the regulator is set a bit conservatively, for safety reasons. So we don't know how much smaller Scott could get before it would be unable to recover. However, note that both times the issue comes up, the same phrase is used:
shrink between the molecules
This would imply that the suit cannot safely shrink smaller than the molecular level. That's a vague definition, because molecules come in a huge range of sizes. However, going "between the molecules" of a solid metal would most likely mean shrinking down to nearly the size of individual atoms.
Unfortunately, this isn't really what Scott
does when he tries it; he simply shrinks small enough to fit
through a seam between two sheets of titanium in the YellowJack suit.
Once there, he's still clearly much bigger than molecular level, since we can see him being the size of typical electronic components -- which are often bigger than ants in the first place.
So, the ultimate conclusion is: the movie gets the molecular-level science pretty badly wrong, so there's no way of knowing exactly what size a person has to reach before the end up "going quantum".