The pebbles should be no bigger than the size of a pea. If, however, someone throws a bigger stone, according to the majority of scholars it will suffice him for completing the rite, but it is disliked.
In this regard, we will cite what Sheikh Sayyed Sabiq states in his well-known book Fiqh Us-Sunnah:
“`Abdur-Rahman At-Taimi said: “The Prophet (peace and blessings be upon him) commanded us in the Farewell Hajj to use small pebbles (the size of a pea)” (At-Tabarani).
The pebbles, as stated in the above hadith, should be no bigger than the size of a pea. That is why the scholars consider this size as desirable. If, however, someone throws a bigger stone, according to the majority of scholars it will suffice him for completing the rite, but it is disliked. Ahmad ibn Hanbal holds that it will not be sufficient for him until he repeats it with pebbles (of suitable size), because the Prophet (peace and blessings be upon him) did so, and he forbade the use of bigger stones.
Sulayman ibn `Amr ibn Al-Ahwas Al-Azdi reported from his mother that she said, “The Prophet (peace and blessings be upon him) was at the bottom of the valley, and he was saying, “O people! Do not kill each other! When you throw pebbles, use pebbles (the size of a pea)” (Abu Dawud).
Ibn `Abbas reported: The Prophet told me, “Come, pick some pebbles for me!” I picked small pebbles like peas. When I gave these pebbles to him, he said, “Use pebbles similar to these (in throwing), and beware of exaggerating in your religion, for those before you were destroyed because of their exaggeration in religion” (Ahmad and An-Nasa’i). The majority of scholars take these hadiths as indicating preference for the size of the pebbles. There is consensus that only pebbles should be thrown and not iron, lead, and so on.
The Hanafi school, however, disagrees and holds that it is permissible to use anything that comes from the earth, including stones, clay, bricks, soil, or ceramics and so on, because the hadith only mentions throwing and does not specify what should be used. The action of the Prophet (peace and blessings be upon him) and his Companions indicates the preference and does not specify anything in particular. The first opinion is preferable because the Prophet (peace and blessings be upon him) used pebbles and he ordered others to throw pebbles the size of peas, which may apply to various kinds of pebbles but not to anything else.”