Spacing on Synopsis

Usually single spaced.

Seconded.
Synopsis = single-spaced
Writing sample = double-spaced

I always double space though I do believe most writers single space.
I've never seen an agent list a preference.

Sat Nam! (literally "Truth Name"--a Sikh greeting)

The agency Dystel & Goderich specifically states on the website to double space everything except the query letter. And they ask for a synopsis. So, go with what the agent says on the website. For those who say nothing, the default seems to be single spaced, but I doubt any would throw it out for double spacing.

Blessings,

Siri Kirpal

Double-spaced unless it exceeds. say, two pages.