There's the jacket blurb/summary: a few paragraphs.
There's the short synopsis: 1-2 pages.
There's the longer synopsis: 3-5 pages.
There's the really long synopsis: 5-15 pages.
Ask the agent when you're not sure.
The following is the summary/blurb of my novel, The Pacific Between:
"Betrayal makes us do strange things."
When Greg Lockland returns to California for his parents' funeral, he discovers letters that suggest an affair between his ex-lover, Lian, and his late father. Suspicions, anger and jealousy take Greg on a transpacific journey to find the truth. One by one, people from the past return to his life, including elusive and perfect Lian. Uncovering deep-rooted deceptions creates more twists and turns to the past than an old Chinese alleyway.
The Pacific Between evinces the power of unconditional love and deals with personal subjects such as death, estrangement, and betrayal. It is a man’s journey to discover himself and the world around him. Set in beautiful Asia and told with wit and humor, this nostalgic tale speaks true to the heart about relationships, families, and sacrifices.