To research loglines...i looked at examples so i could get a rough idea of how to write mine for my film:
Example 1:Title: SWINE HEART HORRORWriters: Theresa Carey and Bruce BrochtrupGenre: Sci-Fi HorrorLogline: A mild-mannered Jewish doctor struggles with bizarre personality changes, unfamiliar slaughterhouse memories and increasingly violent episodes after receiving an emergency heart transplant of unknown origin.
Example 2:Title: Mo MushiWriter: Wade BarryGenre: DramedyLogline: Two gay men from San Francisco move to a small Wisconsin town to open a sushi dance club.
Example 3:Title: KeynoteWriter: Zach AsmanGenre: DramedyLogline: An Internet billionaire returns to his hometown to deliver the keynote speech at his old high school's graduation.
Example 4:Title: Two Can PlayWriter: Glen DelahaveGenre: Black ComedyLogline: After suspecting his wife is cheating, a bitter lawyer treats it like any other case and must begin an investigation to gather witnesses, obtain evidence and arrange for an extravagant dinner party at which to present his case to her family and friends.
From looking at examples i come to the conclusion that loglines are a brief summary of a film often providing both a synopsis of the films plot, this is to connect to the audience and stimulate the audiences interest in the film. Loglines can be a sentence long or a short paragraph, the reason for this is because they are enticing the audience to see their film where as if it was longer that might give away the story making the audience not want to see the movie because the element of surprise has been taken away!