It is narrated from Nafi‘ that Abdullah bin Umar (may Allah be pleased with them both) used to say takbeer at the time of throwing pebbles, for every pebble he threw.

Imam Malik, may Allah have mercy on him, said: I heard from some people of knowledge, they used to say: The pebbles should be such that one can throw them with two fingers.
Imam Malik, may Allah have mercy on him, said: In my opinion, they should be a little bigger than that.