I just finished the first draft of my synopsis for Slideways. So far I've sent two query letters, both to agents who did not request synopses. Unfortunately, almost all the agents on my list require the dread document, so I've been forced to write one.
For those unaware, a synopsis is a mini-encapsulation of your book. But it's not enough to simply recap the major events; it should be dynamic, interesting, and capture the essence of your novel. All in (depending on novel length and agent preference) one to eight pages.
My current synopsis is about 2 1/2 pages, single-spaced. That feels about the right length to me, and fits with the preferences I've been reading on some agents' blog. It still needs work, though. Right now I need to step away, and I'll look at it fresh tomorrow.