In general, there is no specific size limit for something to be considered within the realm of quantum physics. Quantum physics is a branch of physics that describes the behavior of particles and systems at the microscopic scale, where quantum effects become significant.
Quantum mechanics, the foundation of quantum physics, deals with phenomena such as wave-particle duality, superposition, entanglement, and quantum uncertainty. These phenomena are observed in various systems, including atoms, molecules, subatomic particles, and even larger systems like solid-state devices.
While quantum effects are typically more pronounced at the microscopic level, they can also manifest in larger systems under specific conditions. For example, macroscopic systems can exhibit quantum behavior in the field of quantum optics, where light can be manipulated as individual particles (photons) or through quantum entanglement.
Therefore, it's more accurate to say that quantum physics focuses on the study of phenomena governed by quantum mechanics rather than being restricted to a specific size limit. The applicability of quantum principles depends on the characteristics and interactions of the system under investigation rather than the size alone.