There’s an interesting article in “New Advent”, under the tital “Altar Breads”, that contains this note:
“The earliest documentary evidence that the altar-breads were made in thin wafers is the answer which Cardinal Humbert, legate of St. Leo IX, made at the middle of the eleventh century to Michael Cerularius, Patriarch of Constantinople. These wafers were sometimes very large, as from them small pieces were broken for the Communion of the laity, hence the word “particle” for the small host; but smaller ones were used when only the celebrant communicated.”
Further investigation was prevented by time restraints. Hope this helps, somewhat.,