How big was the stone David used to kill Goliath?

After David carefully sized up Goliath, he then grabbed from his purse a smooth stone given to him after recently falling from the sky landing just inches from his left foot.

Once he picked out the stone he immediately knew that its size, weight and especially its shape was the perfect candidate to take the giant Goliath out and off the battlefield indefinitely.

After dodging to and from these enormous spears, one of them just narrowly hitting David's right cheek leaving a three inch gash that began to bleed heavily though poised non life threatening and it was at that point in time that David in a span of just three seconds let loose everything he had and released the smooth stone from his homemade sling hitting the giant sharply in the neck doe to its limit of armor therefore exploiting the best possible opportunity to take Goliath out of the equation but without killing him by showing mercy. If Goliath wanted to die that morning it would of been buy his own hands and not that of David's... Incidentally I possess the smooth stone in question...