The synopsis, or what I generally call a two-pager, is a vital document for a writer because it's just about the most you can usually get anyone to read. This is fair — people are busy — and in fact more and more people (including myself) are now asking people to send a 20-30 word logline when they initially get in touch.
But a two-pager is a manageable length too, as long as there aren't too many to read in a day. Heck, even people who say they only read one-pagers can be convinced to trawl through a two-pager if you ask them nicely. (This doesn't apply to faceless online submissions of course. That would just make you appear difficult or borderline crazy.)
I like the two-page format as it not only allows for a lot more breathing room and nuance than a logline (obviously) but it allows the writer to communicate the structure. The best examples have around eight paragraphs to a side and those paragraphs correspond directly to the act breaks and midpoint.
Writers who spend a page and a half describing the first act and then rush (or omit entirely) the ending are holding up a red flag to a world-weary reader that their script's structure may not be as well-balanced as it needs to be, and you'd be wise to bear in mind that readers have probably seen every trick in the book and are looking for any excuse to not have to read your submission.
A question on the subject of synopses caught my eye in the Quora weekly email this morning, and the top-ranking answer is full of great advice, as are some of the others:—
It doesn't address film synopses specifically, but it's much the same, and the guy's style is entertaining even if he doesn't cover every base.
While you're there on the Quora website, click around some of the related questions in the top right — there's usually some good advice and lots of strongly-held opinions. Warning: as with TV Tropes this website is a time suck. Don't let it get in the way of your actual writing!