What size breasts are required for a girl to be an adult?
According to Halacha a female has three legal stages to her life, based on age: girl, youth, and adult. The legal status is determined by two parameters: age and physical maturity. In terms of age: a female is a girl until the age of twelve, a youth from twelve to twelve and a half years of age, and adult after the age of twelve and a half years. In terms of physical maturity: a female is a girl as long as her breast are immature, a youth when her breasts are as a fruit in the process of ripening, and an adult when her breasts have grown. The sages were divided on the size of an adults breasts. One said that her breasts were large enough that a sort of crease was formed beneath her breasts, and Rabbi Akiva said that her breasts had grown enough that they incline. Another sage said that the aureole around the nipple begins to darken. A different sage said that her breasts are large enough that if one flattens the nipple is takes time to rise again.
One of the scholars, Samuel, clarified the opinion of the sage who said that a sort of wrinkle is seen beneath the breast. This does not mean that the breasts are very large and so a wrinkle is seen, but that if the girl puts her hands behind her back, a wrinkle is noticeable beneath her breasts. To authenticate his idea, Samuel examined the breasts of his 12 and a half year old maid. After he examined her he paid her 400 zuz for having caused her embarrassment by forcing her to expose her breasts to him. Samuel was extremely careful about causing embarrassment to his slaves and maids, so he did not give his maid for strangers to sleep with, only his own slaves. In contrast, Rav Nachman was not careful about causing embarrassment to his maids and would give them to others' workers for sex. Rav Sheshet would even give them to gentiles for sex.
Other sages defined the adult woman's physical maturity needed to determined her legal standing. The sage Rabbi Elazar son of Tzadok ruled that her breasts must be large enough that she could sway them. Another sage, Rabbi Yochanan son of Baroka, said that she is mature when her breasts take on a silver tinge. The scholars asked: Is this not a sign of old age and not newly-gained maturity? Therefore he explained his words as meaning "When the top of her nipple splits" (an unclear definition). Another sage, Rabbi Yossi, said that her breasts must sprout a thick nipple with an aureole around it. Another sage, Rabbi Simeon, gave a clue about the adult woman's physical maturity based on her sexual organ: when the flesh of her sexual organ swells and can be noticeably bent, as though it were a spoon.
(Babylonian Talmud, Tractate Niddah 47a-b)