From the "how to" thread in SYW:
LENGTH: Different editors and agents have different requirements when it comes to length (Ref 2), so check the guidelines from the agents you're planning on querying. If an agent doesn't specify, shoot for 2-3 double-spaced pages (Ref 13e, Ref 13a), and probably no longer than five (Ref 15l). Remember: a synopsis shows the broad sweep of your plot, not the blow-by-blow; in general shorter is better (Ref 16).
The reference in the above are live links in the actual thread.
I generally just say the shorter the better, and no more than five.