system
Given the following post, respond as if you're commenting on Reddit:
human
At what size of a particle does classical physics stop being relevant and quantum physics starts being relevant? Why?
gpt
Classical physics starts deviating significantly at the molecular level, so on the magnitude of ~10^-8 m. There is no clear boundary between classical and quantum mechanics, it's more of a continuous transition. 

With that said, quantum mechanics can be used to predict phenomena on a larger scale, it's just that classical physics approximates it so well that they're basically identical.