Why do ripples formed in water after throwing stone are circular not of the size or shape of stone ?

Ripples are surface waves, and waves are a means of transferring energy.

The shape and speed of the wave are controlled not by the initial disturbance but by the properties of the medium, in this case the water.

The amplitude (strength) of the waves are controlled by the energy of the initial disturbance.

This disturbance of course is the fact that the stone has kinetic energy, most of which it loses when it hits the water and slows right down.

It is this energy which travels outwards as expanding ripples.

Edited October 31, 201312 yr by studiot

The propagation speed tells how far the ripple reaches at a given time, and this makes a circle. The stone's shape has some influence by telling where the ripple starts, but once the ripple is much wider than the stone, this influence is small.