The أُسس السنة is to throw exactly seven pebbles at each Jamarah, saying 'Allahu Akbar' with each throw. This is the consistent practice of النبي (صلى الله عليه وسلم) and is considered واجب (واجب) by all four schools. Throwing fewer than seven pebbles means the stoning for that Jamarah is incomplete, and the missing pebbles must be thrown to complete it.
الشيخ ابن عثيمين explained that if a الحاج is uncertain whether they threw six or seven pebbles, they should throw an additional pebble to be safe, as they build on certainty (the lower number). However, deliberately throwing eight or more pebbles is مكروه (makruh) because it goes against the prophetic practice and النبي said: 'Use pebbles like these' (referring to small stones) and warned against exaggeration. The extra pebble does not invalidate the stoning but is contrary to the السنة.
Regarding the size of pebbles, الشيخ ابن باز specified that they should be slightly larger than a chickpea (approximately the size of a date pit or a small bean). Using large rocks, shoes, or other objects is prohibited, as النبي (صلى الله عليه وسلم) specifically warned against extremism (ghuluw) in this context. He said: 'O people, do not kill yourselves. Use pebbles like these,' indicating small stones. The pebbles should be collected from مزدلفة or anywhere along the way — they do not need to be washed, though some الحجاج prefer to wash them.