Quantum Mechanics is the branch of physics that was invented by Albert Einstein - in fact, he won his Nobel prize for work that became quantum mechanics. Einstein discovered that when things become very, very small, they tend to come in packets which you can count with integers. One of the fundamental properties of these quanta is that they are so small that you can't tell where they are and how fast they are moving. Nobody can use a modern computer or listen to a radio or a CD or see a laser and not be assured that quantum mechanics is real and not mystical. Microscopic - an object is microscopic if its behavior can be understand using quantum mechanical principles.
Molecules are borderline microscopic.
Atoms and subatomic particles certainly are. Although it weighs more than this congregation, it is still a quantum mechanical device. Also, the fact that an iceberg floats in fresh water can only be explained with Quantum Mechanics. Macroscopic - an object is macroscopic if its behavior can be understood using Newtonian mechanical principles. Viruses, bacteria, and anything larger is Macroscopic.