In medical practice, the dimension of the penis that is measured is the length. The measurement is taken along the upper surface of the shaft of the penis to the tip, using a measuring tape or preferably a ruler. The ruler is pressed firmly into the soft tissue over the pubic bone (the symphysis pubis) since, in obese boys and men, a seemingly small penis may be partly engulfed by the fat pad at its base and actually be quite normal in length. Normal standards are available for penile length. True micropenis may reflect failure of normal hormonal stimulation or a failure of normal development during the formation of the embryo (a birth defect).
The prefix "micro-" is derived from the Greek "mikros" meaning small. It diminishes whatever it precedes.