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 holds that it will not be sufficient for him until he repeats it with pebbles (of suitable size), because the Prophet (peace be upon him) practiced it, and he forbade the use of bigger stones.
Sulaiman bin ‘Amr bin Al-Ahwas Al-Azdi reported from his mother that she said, “The Prophet (peace 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.” This was reported by Abu Daw’ud.
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 against exaggerating in your religion, for those before you were destroyed because of their exaggeration in religion.” This was reported by Ahmad and Nasa’i, its chain of authorities being sound. The majority of scholars take these ahadith as indicating preference for the size of pebbles. There is consensus that only stones (pebbles) be thrown and not iron, lead etc.
The Hanafi school, however, disagrees and holds that it is permissible to use anything that comes from the earth, stones, clay, bricks, soil, or ceramics etc., because the hadith mentions throwing only (and does not specify as to what should be used). The practice of the Prophet (peace 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 be upon him) used pebbles and he ordered others to throw pebbles like peas, which may apply to various kinds of pebbles but not to anything else.