Corrected entry: After Hooper cuts open the Tiger shark, he tells Brody that it isn't the shark. He adds "you still got a hell of a fish out there, with a mouth about this big." He indicates size of the bite by holding his hands about 2 feet apart. This estimate is based on the bite marks on the first victim. Of course, later we see the shark's bite radius is much larger, around 3 to 4 feet. Hooper is a shark expert and should be able to more accurately determine bite radius. (00:44:30)
djm
24th Jan 2017
Jaws (1975)
Correction: All that was left of the one and only victim that he examined, Chrissy Watkins, were pieces so effectively you could argue there were no bite marks, and he was making an assessment of best judgment and experience. Either way, 2 feet is still a big mouth.
Correction: Clearly nobody involved, including Hooper, believed a shark as large as the one later seen could exist. His estimate was based on what he concluded could be the largest shark out there.
Hooper is a shark expert. His bite radius estimate should be based on the available facts, like the actual bite marks in the victim, and not what he thought would be the biggest shark. It's pretty difficult the get the bite radius wrong by a foot or more when you have clear bite marks in the victim.
djm
Unless a bite radius can be estimated by the distance between puncture marks, Hooper had no way of knowing how large the shark actually is. As the previous correction states, he's using his own knowledge and experience to guess the size.