Multiple astronomers were hired to serve as consultants on “Deep Impact,” employed to assure the comet science would be as accurate as possible. The team included Gene Shoemaker — the co-discoverer of the Shoemaker-Levy 9 comet — astronaut David Walker, Chris Luchini, and Joshua Colwell, a physics professor at the University of Central Florida. Colwell noted that: 

“It’s not hard to be more scientifically accurate than most sci-fi movies. […] [The] director, producers, and writers made a decision to make the movie as realistic as possible while staying true to the story they were telling. […] The movie depicts both an attempt to deflect the comet and also the creation of a subterranean ‘ark’ to house a large number of people to survive the catastrophic and long-lasting effects of the impact. […] Both of these activities are plausible, but both require huge resources and a lot of time to put together.”

Colwell and the other advisors also made sure the surface of the comet looked correct, and that it was a scientifically accurate size; in the case of the movie, it was seven miles across. They also wanted to make sure that the strike looked like an actual comet strike might, posited what would happen to Earth’s oceans (there is a massive, city-swallowing tidal wave), and were careful to show that any astronauts who visited a comet would still be weightless while close to its surface. 

For the record, any heavenly body with gravity would necessarily, thanks to a process called isostatic adjustment, be spherical. The smallest spherical body with its own gravity in the solar system is Mimas, the seventh-largest moon of Saturn, which is about 246 miles in diameter. Meanwhile, the largest comet known is C/2014 UN₂₇₁, which is only about 1.2 miles in diameter.