From a story on lithub.com by Michael Schulman headlined “How a 26-Year-Old Steven Spielberg Made Jaws…and Nearly Lost His Career in the Process”:

In late 1973, Steven Spielberg called a meeting to figure out just how on God’s green earth he could make a movie starring a shark. Production designer Joe Alves, an ex-race car driver, had consulted ichthyologists, who said that the biggest great white sharks were, at most, 19 feet long, and untrainable. But the shark in Peter Benchley’s (still-unpublished) novel was a whopping 25 feet—which would require a lifelike mechanical monster that could swim and thrash in the open ocean. Nothing like that had ever been done.