The largest reptile known to have existed was a South American sauropod called argentinosaurus. According to Bob Strauss, writing for About.com, other giant sauropods are hinted at in the fossil record, but the dimensions of the argentinosaurus are well documented. It was 120 feet long and weighed 100 tons.
The largest living reptile is the estuarine, or saltwater, crocodile. The saltwater crocodile is indigenous to southern Asia and Australia, and it is known to grow up to 20 feet long and reach a weight of as much as 3,000 pounds. Recovered crocodile remains suggest a maximum length of over 21 feet.