The best description I ever got of a synopsis was from a workshop mentor who said, "Imagine two teenage boys coming out of a movie, recounting the plot to one another." It's all present tense, it's short sentences and paragraphs, it's all the high points without much detail.
Length is highly variable. I sold, then wrote, Trial of Flowers off a five paragraph synopsis. I know people who routinely write 50-75 pages of synopsis before they consider themselves ready to write the novel. Some of that has to do with how much detail you want or need to capture.
Remember that a synopsis serves multiple purposes. It's a selling document, to promote your work to your agent and your editor, and once sold, to the marketing department. It's also your roadmap while writing the book. (Sometimes a synopsis written for that purpose is called an outline.)
Here's something to try. Write a long synopsis, 5-10 pages or more, with a lot of detail. Then write a short one, 1-2 pages. Then write a 1-3 paragraph description, as it might be explained in a review. Then write a 1-2 sentence description, as it might be explained in a publisher's catalog listing. If you can do all of those things, then you really understand your book. As a practical matter, you might not be able to do all of those things until after you've written the book, but you should probably be able to do some of them.
Don't be coy. This isn't the place to hide the Big Secret. An agent or editor reading it needs to know how the story comes out, who the secret heir is, where Uncle Wally hid the atom bomb. Put everything in there that needs to be there.
Don't be rigid. No battle plan survives contact with the enemy. No synopsis survives contact with the manuscript. On the one hand, use it as your guide. On the other hand, if you have a better idea while you're writing, follow the muse.
One thing I do is paste the synopsis at the bottom of my working draft. Then, as I go through the novel (since I write them front-to-back, generally), I delete portions of the synopsis which have been filled in. In essence, I use it as an outline in the term paper sense, or at least as a writing guide.
If you're curious, here's a link to the synopses of Mainspring [ Powell's | Amazon ] and Trial of Flowers [ Powell's | Amazon ].
This will be a lot more useful if you've read the books, I should think, but feel free to look at them in any case. Remember that Mainspring sold as a finished spec book, while Trial sold off the synopsis you see there.
Once again, feel free to make with the comments, corrections and alternatives.
ETA: pnh, who knows whereof he speaks in glittering spades, has added the following two corrections:
Trivially, our catalog descriptions are usually a bit longer than 1-2 sentences. Depending on the book, they run from three sentences to three or four paragraphs.
More importantly, catalog descriptions, front flap copy, and the like--all of which I call "story copy"--is fundamentally different from synopses and other working documents. The purpose of story copy is to provide the setup while withholding the resolution. It's the movie trailer, not the two kids explaining the movie to each other after they've seen it.