The right answer is the size.
Crystallization is the operation that consists, of a solution (solvent + solute) or a molten solid, to form a crystallized solid. The solute crystallizes in general in a predefined geometrical form (cubic, face-centered cubic, etc.), including or not solvent molecules (eg pure NA2SO4 or [Na2SO4, 10H2O]). The form or chemical formula of the crystals may depend on the crystallization temperature.
There are two ways of crystallization: The dry way and the wet way (in solution)
In crystallization in solution: the solute is initially in the liquid phase in a solvent. It is crystallized (solidification ordered according to a regular structure) within the solution either by solvent evaporation, or by cooling the solution, or both.