Why there are ground rods made in different
sizes?

Different areas have different requirements.

Different soils are harder to bang a rod into.

A supplimentary rod need not be as long as a primary rod.

It mattters whether the rod is solid copper, or just copper covered steel.

The rods come in different sizes in order to facilitate the variety of needed installations. Eight foot rods are the minimum length that the code allows but the specifications on the job may call for an identified impedance to ground that can only be achieved with stacked rods or rod arrays arranged over a wide area. When rods are to be stacked or coupled to achieve a greater depth it is obviously less expensive to use ten foot rods coupled together than it would be to use eight foot rods. Extremely low ground impedance values can often be achieved only with deep driven rods of forty or more feet in length.

800.100(B)(2)(2) allows a five foot ground rod for communications.