Simplify the following text for a Grade 8 native English-speaking person.
Original: All but the leatherback are in the family Chelonioidea ; the leatherback belongs to the family Dermochelyidae and is its only member , the leatherback sea turtle , is the largest , measuring six or seven feet ( 2 m ) in length at maturity , and three to five feet ( 1 to 1.5 m ) in width , weighing up to 1300 pounds ( 600 kg ) .
Simplified: All species except the leatherback are in the family Cheloniidae . The leatherback belongs to the family Dermochelyidae and is its only member .