The length of a synopsis can vary, and usually the producer will tell you what length s/he wants. If not, do a one-page synopsis. I've never seen a request for a synopsis longer than three pages. Also, the length preference often is a maximum length. So I suggest writing the one-page synopsis.
Write your synopsis in the present tense (with very little or no dialogue) and give the highlights of your story. Keep the focus on the protagonist and on the hero's character arc and journey. Write simply.
Example of a synopsis of a script that I optioned. (This later was rewritten with the producer in a major way: both scripts are available for comparison, along with my comments.)
Jerrol LeBaron, President of Writers' Script Network, an online marketing company, makes this recommendation for testing the simplicity of your synopsis:
"Get your synopsis read by some teens. If those teens aren't able to read your synopsis and instantly know what your story is about, your synopsis is in trouble. If the teens had to read a particular sentence or paragraph (in your synopsis) again to understand it, your synopsis is in trouble. When they instantly (and I do mean 'instantly from one quick read') know what your story is about, you have a good synopsis. If those teens are confused about your story in any way, rewrite your synopsis."Scripts are where movies come from, but the industry determines their interest in a script by the pitch, logline and synopsis. Your logline and synopsis are the key that opens the gate to getting your script read. The pitch (verbal form of a logline) is THE tool that a producer uses to get a film financed. If you, the writer of the script, can't effectively communicate what your story is about, how can anyone possibly expect a producer to come up with a pitch to get the film financed?
"If a teen can't understand your story from the synopsis or logline, neither will the development exec. or producer. This has nothing to do with intelligence. It has everything to do with how hectic it is for producers and studios. These people have to explain your story to somebody else. The easier you make it for them, the better your chances."
More and more, producers seem to be asking for a short synopsis; it might be a good idea to include one with your script as a matter of course.