How Big a Particle?

I understand that the famous Double-slit experiment has been done with other particles besides photons that behave as both particles and waves. Today I wondered if anything larger had been tried. Apparently a paper was published in 2002 entitled "Quantum interference experiments with large molecules" where C60, fullerene molecules, were used in place of photons and it was found they behaved the same way. Where, exactly, does quantum behavior break over into macro behavior?