How big a system can be while maintaining a quantum entanglement (also called 'quantum coherence') - is still an open question. A very important one, because it

How big a system can be while maintaining a quantum entanglement (also called 'quantum coherence') - is still an open question. A very important one, because it affects quantum computers. Experimentally researchers see, that a big enough system can't maintain quantum coherence for long. If you want to know about how this process is described mathematically, search for 'quantum decoherence'. If you want to know about possible physical interpretations, search for 'objective collapse theory'.