As I am talking about 'smallest' can I expect that it should be a quantum system? I understand that we use quantum chaos theory instead of perturbation theory when the perturbation is not small. For example when quantum numbers are very high for an EM field interacting with an atom, we cannot use perturbation theory. In this case we will use the theory of quantum chaos.
My question is what is the lower boundary of the size of the system or more specifically of the quantum number in case of a EM-field atom interaction for which we must use quantum chaos theory?