quantum mechanics - What's the boundary of microscopic world and macroscopic world?

In other words, what's the maximum size of a Quantum denizen upto which it shows Quantum behaviors? How big do I need to create a molecule (or, collection of molecules) so that Feynman's multiple path and multiple history would no longer be applicable to it? In a sense, Feynman's multiple path and multiple history should be applicable to macroscopic system producing Quantum Decoherence, but I am talking about a molecule which could show interference with Multi-slit Diffraction Experiments.