some info on the ACS website.

Adjuvant systemic therapy (chemo and other drugs)
For women who have a hormone receptor-positive (ER-positive or PR-positive) breast cancer, most doctors will recommend hormone therapy (tamoxifen or an aromatase inhibitor, or one followed by the other) as an adjuvant (additional) treatment, no matter how small the tumor is. Women with tumors larger than 0.5 cm (about ¼ inch) across may be more likely to benefit from it. Hormone therapy is typically given for at least 5 years.

If the tumor is larger than 1 cm (about ½ inch) across, adjuvant chemotherapy (chemo) is sometimes recommended. A woman's age at the time of diagnosis may help in deciding if chemo should be offered or not. Some doctors may suggest chemo for smaller tumors as well, especially if they have any unfavorable features (a cancer that is growing fast; hormone receptor-negative, HER2-positive; or having a high score on a gene panel such as Oncotype DX).

For HER2-positive cancers, 6 months to a year of adjuvant trastuzumab (Herceptin) is usually recommended as well.

Sauce：https://www.cancer.org/cancer/breast-cancer/treatment/treatment-of-breast-cancer-by-stage/treatment-of-breast-cancer-stages-i-iii.html