Don't go with a smaller font, that will make it harder to read. Single-spaced should be fine (unless the person you're sending it to tells you otherwise) -- the synopsis is to be read, not marked up by an editor.
Rather than repeat what I've said elsewhere about what goes in the body of the synopsis, I'll just direct you here:
Rumblings (and Ramblings) at Madeline's -- JANUARY 2008
(It's part way down the page.)
Anyway, this is how I write a synopsis, and it's been successful for me.