How brief is brief, exactly? One page? One and a half?
I've heard that when it comes to agents, the shorter the synopsis the better. For those of you that may not know, a synopsis is a summary of your manuscript. It's like the blurb on the back of book, only to my understanding, it's supposed to be simple and concise, describing the bones of the story, and it MUST include the ending.
Now, how do you summarize the events of hundreds of pages into a single page--double-spaced? It's important to cut out unnecessary words. My suggestion is to start from the beginning--well maybe when the main characters meet, and be sure to try to limit your descriptions of side characters. Introduce the conflict, describe their struggle to resolve the conflict, and be sure to include how they resolve it and what happens afterward. Include any major twists.
I'm no guru on the subject; I've only written two brief synopsizes, and my most current one didn't quite fit onto one-page. But since the agency didn't specify a single page, I sent it anyway, although in retrospect I probably should have revised it more. It's difficult to cut everything down to the bones, especially when there's so much story the author feels is important. But, keep practicing!
Now, I won't include my synopsis, simply because mine went over what I would consider brief, and I don't want to be leading anyone astray. But, I will include a wonderful website with plenty of samples that really helped me: