For one thing, it depends on how you work. Maybe you're the person who writes 100 pages of notes and very very detailed outline as you start a project. Then your synopsis is 100 pages, and you've satisfied yourself that the book is viable and you can write it. From that you can probably pare the thing down to conform with the editor's request. And that's the second thing: It depends on what the editor is looking for. Some of them want five pages. Some want 10 or 20 but no more than 25. So the real answer, then, is find out what the editor wants and write a synopsis to more or less conform.
I've sold books now on the basis of an oral description, a nine page outline, an 18 page detailed synopsis, and on 150 pages plus a brief five page synopsis of what else happens.
That said, I also find writing the synopsis to be the most hateful aspect of the process, because in many ways it feels vampiric. It's sucking the life out of a story that hasn't been told (i.e., written) yet. I get the same feeling of story enervation from telling people about the work in progress at conventions. People always want to know what you're working on, but sometimes I think the story is too fragile to be taken out of the incubator. If you do it too much, you'll tire of the idea.
Honestly, though, you look at a dozen writers' perspective, you'll find someone who just loves to write synopses. It is, as with all things in writing, a matter of how you work, which won't match anyone else's method point for point, and to some degree you just shouldn't worry about it very much.
All the same, having said that, I'm attaching here, in PDF format, a link to the final synopsis for my novel FITCHER'S BRIDES. Having been invited by Terri Windling to write a book in her Fairy Tale series for Tor Books, I worked out my version of it and sent her what remains the most detailed synopsis I've turned in to date. It sold the book. But in many ways it was a structural outline both for her and for me. May it prove useful to someone wrestling with this dilemma. Those familiar with Fitcher's Brides may find the differences interesting, too...which points to the fact that, no matter how detailed the outline, in the writing of it things change.