There is a relationship that determines indeterminism :). It is called the Heisenberg Uncertainty Principle.
Size can be described by the variable $x$ for the position of a particle/atom/molecule. The principle says that we can only know the value of $x+\Delta x$ and the momentum of the particle $p+\Delta p$ (where $\Delta x,\Delta p$ denote small intervals) within a relationship bound by
$$\Delta x\cdot\Delta p \gt \hbar/2$$
where $\hbar$ is the reduced Planck constant
This means that if we want great accuracy in position the momentum will be indeterminate.
Equally if we want great accuracy in Energy, Time will be indeterminate.
From the wiki link:
the uncertainty principle actually states a fundamental property of quantum systems, and is not a statement about the observational success of current technology.It must be emphasized that measurement does not mean only a process in which a physicist-observer takes part, but rather any interaction between classical and quantum objects regardless of any observer.
There is no unique border, it depends on the variables under observation, but
ħ is a very small number, which can be approximated with 0 in the macroscopic world. The Heisenberg uncertainty is relevant for the mircoscopic world of atoms and molecules and smaller.