However, if we're talking about the largest reptile that ever lived, the answer is the apatosaurus, a dinosaur that could grow up to 110 feet long and weigh over 100 tons.
It's important to note that dinosaurs are not reptiles. They are a separate group of animals that evolved from a different branch of the evolutionary tree. But since the question asks about a giant reptile, I included both the largest living reptile and the largest extinct one that fits that category.