It depends what sort of book you're writing the synopsis for. If it's non-fiction: I'd say synopses/proposals are usually anything between two and eight pages, give an outline of the book, an indication of why the book is needed, refers in some way to the competition, and tells why the writer is the right person to write this particular book.
However, for fiction, none of that stuff is needed: just outline the story in present tense, single-spaced, and yes, you DO need to give away the ending! There's a good site here:
I hope that's a help.