Nabi (shallallahu alaihi wa sallam) described the correct pebble size as 'the pebbles used for slingshots' — roughly the size of a chickpea, date pit, or small marble. In modern measurements, this is approximately 1-2 cm in diameter. The pebbles should be natural stones, not manufactured materials. Nabi explicitly warned against extremism in the stoning, indicating that excessively large stones are not appropriate.
Common mistakes to avoid: using large rocks or stones (which can seriously injure other jamaah haji in the crowded area), using very tiny pebbles or sand grains (which may not fulfill the requirement), throwing shoes, sandals, or personal items (which some frustrated jamaah haji have done historically), or using pieces of concrete, brick, wood, or metal. These materials do not fulfill the Sunnah requirement of using natural ground stones.
The pebbles do not need to be from any specific location — they can be collected from Muzdalifah, Mina, or anywhere else. They should be clean if possible, though washing is sunnah, not required. There is no requirement that pebbles be of a specific color or type of stone. Simply pick natural small stones from the ground that match the chickpea-to-date-pit size range. Carrying a few extra pebbles is wise in case you drop some during the stoning or are unsure whether a throw landed correctly.