In general, a logline should be about 20 words long and should capture your storyline.
The problem is that you rarely see actually loglines that short! Here's one I saw as a sample on ScriptShark:
A college freshman girl's arrival to campus spawns mysterious killings revolving around the football team.Okay, from that we know the protagonist, where it takes place, and that it's probably horror ("mysterious killings", "spawns"). But we don't know what the protag's goal is or who the antagonist is. It fits the word count, but, in my mind, it's not complete.
Here are a couple of more (and I'm sorry to say that I've forgotten where I gathered these):
A playboy manufacturer rescues 1,100 Jews from certain death. Appalled by atrocities in Nazi Germany, he hoodwinks the Nazi brass and converts his factory into a refuge for Jews. Based on Oskar Schindler's true story.Both of those are over 20 words and the second sample only implies the goal. But both are compelling and would be hard for someone to pass up. (And they didn't, since they're both produced movies.)
A conscientious sheriff relinquishes his gun and job to marry a pacifist young woman, but on the way to the honeymoon they pass a band of outlaws riding toward their peaceful village to take it over.
It's good to include the protagonist (and goal), antagonist (and goal), and the big disaster or turning point. But it doesn't have to be straightforward.
Screenwriters often take their script and condense it into a 10 page synopsis, then squeeze that to 3 pages, then 3 paragraphs, then 20 words. Give it a try on your book. And remember, make it irresistible and complete.
No problem. Right?