At what size do particles stop exhibiting quantum behavior?
So there is this bunch of scientists who are actually trying to scale up and see what are the largest particles at which they can actually observe quantum superposition. This is really fascinating research. I've added a couple of links here, one to an article in Quantum magazine that explains that research, and another link to their website where this group is working. And they have a lot more follow up articles there as well. Definitely worth reading up if you're into that sort of thing