Miss Snark: This may be a nitwitted question, but should a synopsis be single or double spaced when it is sent with a query letter?
This is not a nitwit question. If you're trying for the nitwit sweepstakes, you have to ask if Miss Snark has a day job.
Synopsis are traditionally single spaced. Double lines between paragraphs. Indents not needed. Go easy on big blocks of text. Shorter paragraphs are easier to read than long ones in this form. One inch margins all around, and in a readable font.
If you are tempted to tweak any of this cause you can't fit all your words on one or two pages, I STRONGLY urge you to whittle your word count before you start dropping your font to 8point.
Making something physically easier to read is just smart. Making it harder to read...well, that's the nitwit strategy.