Tolfalas was an island situated in the Bay of Belfales, some 35 miles southwest of the EthirAnduin. According to the map provided with The Lord of the Rings it is 70 miles long, and 35 miles at the widest.
Very little has been written concerning Tolfalas. At one point, Tolkien wrote that Tolfalas was much damaged in the tumult that followed the downfall of Númenor, but this did not make it to the published Silmarillion.
And while working on The Lord of the Rings, Tolkien once wrote that
but this didn't make it to the final manuscript either. Nevertheless, the isle was drawn on Tolkien's personal working-map already from the original, according to Christopher Tolkien.