Roughly speaking, for modern Great whites, for every inch of an anterior tooth, the shark will have 8-10 feet of length. So a great white shark with 2" anterior teeth will be around 20 feet long. The theory is that since megalodon teeth are similar in shape to Great Whites, then that formula can be used for estimating the length of a megalodon shark given the tooth size.
So the diagram that Dave posted is probably pretty accurate.
So am I in the running for the Golden Boa "Most Off Topic" award? 
Thanks,
Ed
>>im having a hard time trying to comprehend how a shark with a 7 1/2 tooth would compare to a monster shark of today... have any more stats or even better pictures comparing?
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>>-Stephen-
>>-Step-
>>-Steve Lightning-
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