Question :What was the amount of a Sa` in handfuls as was used by the Messenger (peace be upon him)?
Answer : What has been verified for us regarding the Prophetic Sa` is that it is four handfuls of a moderate-sized man. This is what was mentioned by some Muslim scholars such as Ibn Al-Athir in his book Al-Nihayah and Al-Fayruzabady in his book Al-Qamus. As for the Sa`s which we find in markets or Masjids (mosques), as mentioned by the questioner, they differ regarding their quantity. Thus, the estimation of a Sa` should rely on what was mentioned by scholars as explained earlier and Allah knows best.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.