The standard shape: when [inciting incident], a [flawed protagonist] must [goal] or [stakes]. The value is not in pitching, it is in diagnosis. A logline that cannot be written is almost always a story with no antagonist or no stakes, and finding that out in a sentence is cheaper than finding it out in a draft.
Two failures are common. Writing a premise instead of a logline — "a film about loneliness in cities" is a subject, not a story. And hiding the ending — a logline is not a trailer and coyness in it usually conceals not knowing.
For commercial work the logline is also the alignment document. If the client and the studio can both sign the same sentence, most of the arguments later in the process do not happen.
What is a logline?
A single sentence naming the protagonist, the goal, the opposition and the stakes. Its main use is diagnostic: if you cannot write one, the story is usually missing an antagonist or a cost.
How long should a logline be?
One sentence, usually under forty words. Length is not the point — the point is that all four elements are present and specific.