There will be a point when the substance loses its solid property.
This sounds like a version of the Sorites Paradox in philosophy: If you keep removing grains of sand from a heap of sand, at what point is it no longer "a heap"? Or if removing 1 grain can't ever be the difference between a heap and a non-heap, doesn't that imply that 1 grain of sand is still a heap if you repeat the process until then?
There's only a problem if you insist on binary logic, which by definition has hard cutoffs and no fuzzy boundary, leading to non-intuitive results for qualitative descriptions of quantity such as the English word "heap". Fuzzy logic (this small collection of grains "is a heap" is only 0.25 true, 0.75 false) is closer to how humans think, and is (IMO) the most sensible way to resolve the paradox.
Applied to your question, we see that you've created a false premise of there being a sharp cutoff point.
In reality, assumptions (and formulae) that work for large solid objects work less and less well on smaller objects, with probably some aspects "stopping working" sooner than others.
e.g. I think momentum and kinetic energy continue to work until your object is so small that Heisenberg uncertainty is significant. For kinetic energy, maybe when it's hard to make a distinction between kinetic energy vs. thermal energy. But I'm not an expert in this area of physics. @Kai commented under another answer that https://en.wikipedia.org/wiki/Mesoscopic_physics is a separate subject between solid-state and particle physics.