To find out if a solution is saturated, unsaturated or supersaturated, we need to put a crystal or soluble solute in the solution.
An unsaturated solution tends to dissolve more of the solute in it because of which the crystal or solute will reduce in size. This is because unsaturated solutions have weaker bonds also that immediately breaks to make a bond with the newer substance.
A saturated solution can dissolve no more of the solute. Hence, the size of the crystal will remain the same.
Now, in the case of a supersaturated solution, there is an excess of solute in the solution so, it wants to leave its excess solute, due to which added crystal or solute will become bigger in size than before.