The Megalodon's size is determined by the size of its tooth. The Megalodon's weight is determined by it's length according to several formulas.
They lived in the Mioune and Plioune eras about 1.6 million years ago. The megalodon can weigh 47 to 103 metric tons ;sharks eat about two percent in body weight. Scientist estimate that the Megalodon could be up to 50 to 100 feet in length.
http://www.greatsouth.net/fossil_replicas/FR182_Megalodon_Shark_Tooth.htm
http://www.enchantedlearning.com/subjects/sharks/glossary/Megalodon.shtml
http://en.wikipedia.org/wiki/Megalodon#Size_estimation
