فَيَرْمِيهَا مِنْ بَطْنِ الْوَادِي بِسَبْعِ حَصَيَاتٍ مِثْلِ الْخَذْفِ.
Translation: He stones it from the valley floor using seven small pebbles of the khadhf type.
The text specifies two different things: seven pebbles and their small size. Khadhf evokes the sort of small pebble flicked with the fingers, not large stones or unrelated objects.
Min baṭn al-wādī describes the historical approach from the valley floor. Modern multilevel structures and official pathways must be respected; it is not an instruction to enter an unsafe route.