The average testicle of an adult male is 2 to 3 inches long and 1 inch wide. Testicles increase in size during puberty, from a little under 1 inch long to their adult size at about age 16 or 17.
Testicles are gonads that produce sperm and relatively high levels of testosterone. These egg-shaped organs simultaneously fuel and affect the hormonal surges of puberty. A testicle size of under 1.3 inches is unusually small for an adult, but fairly normal for a teenager. Drastic changes in testicle size during adulthood are an indication of cancer.
Some other conditions besides cancer also affect adult testicle size. For example, Klinefelter's Syndrome, a condition caused by an excessive number of X chromosomes, can cause smaller-than-average testicles.