A direct mail postcard has about three seconds to earn attention from someone standing over a recycling bin. The difference between a campaign that pulls and one that gets ignored is almost always in the design and the offer, not the size of the mailing list. A 5,000-home mailing of a bad postcard is just a more expensive way to get tossed.
Most of what follows is obvious in retrospect. Most of it gets ignored anyway, because writing a clear postcard is harder than writing a busy one. The rules below are the ones that hold up across every campaign I run.