A high childhood body mass index (BMI) may be protective against benign breast disease (BBD), but little is known about the effects of other early life body size measures. Thus, we examined associations between birthweight, childhood BMI, height, and pubertal timing and BBD risks. We included 171,272 girls, born from 1930 to 1996, from the Copenhagen School Health Records Register, which contains information on birthweight, childhood anthropometry (7–13 years), age at onset of the growth spurt (OGS), and peak height velocity (PHV). During follow-up, 9361 BBD cases (15–50 years) were registered in the Danish National Patient Register. Hazard ratios (HR) and 95% confidence intervals (CI) were estimated by Cox regressions. At all childhood ages, BMI was inversely but non-linearly associated with BBD. The association was slightly stronger in magnitude for BMI z-scores above 0 (HR_{age 7} = 0.86; 95%CI: 0.83–0.90 per z-score) than below 0 (HR_{age 7} = 0.95; 95%CI 0.91–0.99 per z-score). Associations between childhood height and BBD differed by age; at 7 years the association was an inverted U-shape, whereas at 13 years height was not associated with BBD. Ages at OGS and PHV were positively associated with BBD. Low and high birthweights were associated with lower BBD risks. Conclusion: A high childhood BMI, a short or tall stature at young childhood ages, an early pubertal onset, and low or high birthweights are associated with reduced risks of BBD. These complex associations suggest that the role of these factors in breast tissue development during early life warrants further investigation in relation to BBD etiology.What is Known:• Benign breast disease (BBD) is common and may be an intermediary marker of breast cancer risks.• Early life body size may relate to the development of BBD, but currently little is known.What is New:• Girls with a high body mass index at school ages or with an early pubertal timing have decreased risks of BBD.• Short and tall heights at young childhood ages and low and high birthweights are associated with lower BBD risks.