"Five by Five"

Where does the phrase "Five by Five" come from?

FIVE BY FIVE -- Five-by-five can mean an obese person. But the meaning that's been discussed here at Phrase Finder is from radio communications. Five by five, or "five by," means "I'm hearing you loud and clear." It can also mean, OK or everything's perfect. These definitions are from Random House Historical Dictionary of American Slang, Volume 1, A-G, by J.E. Lightner. I've also heard the term used for describing the bed of a pickup truck that's five by five.