That would be if you're writing a query letter.
An actual full synopsis (which some agents will ask for) is a summary of the entire book from start to finish including all main characters, main plot points, and ending (think about the book reports you read in high school).
There's no hard and fast rule about length but 2-4 double spaced pages seems to be quite typical. Agent Nathan Bransford (gosh that sounds like there should be a double 0 something at the end) has a rather good blog entry on the topic
which can be found here.
There are also lost of samples (with feedback) in the
Share Your Work forums.