Advice on synopsis writing?

Hi, folks, just about ready to submit my second novel to agents, but I want to get better at the submission process. My first novel’s synopsis was about two pages long, but I’ve read advice ranging from “keep it short” to “write at least 20 pages”. Frankly, I doubt an agent would bother to read 20 pages. But if anyone has advice on best way to go about writing a good (I mean “great”, of course) synopsis, from length to style, I would appreciate it.

Try to find out their requirements…

… but if in doubt, between four and ten pages, double-spaced, one-inch margin, present tense.