A synopsis is anywhere from one to five pages, with one, three, and five pages being the three most common. If an agent or editor says "Query with synopsis", it usually means a synopsis that is separate from the query letter itself. Synopses are usually single-spaced, but make sure there's white space on the page. Dense prose seldom looks inviting, even if it's good.
I haven't encountered an agent or editor who objected to a three page synopsis, as long as it looked inviting, and many are fine with five pages. I wouldn't go beyond three, and one page is better. I would, however, include the first three pages of the manuscript, unless the guidelines specifically said not to do so. Really good first pages can salvage even a poor query and synopsis, and as Miss Snark always said, including these pages is an unwritten rule.