I wanted to post what I think is the best information on testicle size.
Referencing the AACE Guideline to Hypogonadism (2002 edition)
Pre-Pubertal testes are betwen 3 and 4 ml in volume and less then 2 cm long.
Peripubertal testes are between 4 and 15 ml in volume and more then 2 cm long.
Adult testes are usually between 20 and 30 ml in volume and from 4.5 to 6.5 cm long by 2.8 to 3.3 cm wide.