I'm thinking about where the classical limit would begin in terms of a particle's rest mass. If we start at the smallest mass scales consisting of subatomic particles, we would have to use quantum mechanics (due to the significant wave nature of particles with small momenta). If we consider heavier things such as atoms, and some molecules, we can still use quantum mechanics. If we go farther into macroscopic things such as a speck of sand, a baseball, etc, then we use classical mechanics. Exactly where do we stop using classical mechanics. Can bacteria qualify as classical particles to a good approximation? What about smaller things like viruses? Is classical mechanics good enough or is too innacurate?