St Mary's is the largest and most populous island of the Isles of Scilly, an archipelago off the southwest coast of Cornwall in England, United Kingdom. St Mary's has an area of 6.58 square kilometres — 40 percent of the total land area of the Isles of Scilly — this includes four small tidal islands which connect with St Mary's at low tide: Toll's Island, Taylor'…St Mary's is the largest and most populous island of the Isles of Scilly, an archipelago off the southwest coast of Cornwall in England, United Kingdom. St Mary's has an area of 6.58 square kilometres — 40 percent of the total land area of the Isles of Scilly — this includes four small tidal islands which connect with St Mary's at low tide: Toll's Island, Taylor's Island, Newford Island and the island at Innisidgen.