Dick should be measured in volume. Think about it. If someone has a 20cm dick but only like 3cm in diameter its not that much dick. I get it would be harder to measure. You could probably use the formula of a truncted cone or cylinder.
If you fill a measuring jug full, put your dick in it so it overflows and measure how much water displaced you could find the volume of your dick that way. That’s how archimedes would do it