Usually, the water moves in and out of cell by diffusion. Osmosis represents the movement of water through a partially permeable membrane and it occurs when the water movement is determined by the different concentrations of solutions found on both sides of membrane.
Hence, the water found in a less concentrated solution moves toward the solution having an increased concentration, as a response to the differences in molarity across membrane. The solutions on both sides of membrane are classified as isotonic, hypotonic and hypertonic, depending on the concentrations of solutes. If solute concentration is equalized on both sides of membrane, the water flow stops.
It is important to notice that the size of particles in solution does not have any influence over the osmosis process.