Based on careful analysis of that vertebral column, they determined that the newborns were 13 feet long. Because science.
Many shark species engage in in utero cannibalism, where the unborn sharks eat each other until only one is left, and that one gets born. That one had plenty to eat in utero, so, they could get quite large before being born.
Newborn great white sharks are about 4–5 ft (1.2–1.5 m) long.