The full grown size of the shark depends of the breed. The whale shark is the largest shark and grows to almost 40 feet long. The great white can reach up to 20 feet long, and bull and tiger sharks grow to 11-14 feet long. You can find more information here: http://www.sharks-world.com/how_many_feet_long_is_a_shark.html
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The size of a shark will depend greatly on the species of shark that you are looking at. There are some small sand sharks that are only a few feet long whereas a whale shark can reach 41.5 feet long.