Dear Miss Snark,
I love your site.
Whare are synopses single-spaced? I would think that the rationale for double-spacing manuscripts would equally apply for synopses.
The ones I see that tend to go for more than a page are double spaced (or my favorite break the rules manuever: 1.5 lines).
Just make it easy to read. This isn't a contest to put one over on an agent. You can do anything you want to, it's not against the law to send tripled spaced, italic font, lambie pie stationery. It makes you look like a nitwit, that's all. The Cover Letter Police are busy steaming stamps off the SASE; they don't have time to actually read what you send.
When I look at a big splat of type on a page, I just don't want to read it. Do yourself a favor: make me reject your writing cause it sux, not cause your synopsis looks like a Rorshach test.