Arbitrary (and usually incorrect) units of measurement used mostly by males. Derives from men overestimating their penis size. Dick inches are much shorter than actual inches. Hence a guy can claim to have a 9 inch penis when it is actually closer to 5-6 inches. The term "dick inches" is usually used when over-estimating non-penis measurements.
> fellow one: "Finally! There's a parking spot!"
> fellow two: "No way, man...You can't park within 30 feet of a stop sign."
> fellow one: "There's plenty of room."
> fellow two: "Yeah, only if you're measuring in dick inches."