Obesity is a controversial risk factor for colorectal cancer (CRC) in older women. We evaluated associations between multiple body size parameters and incident CRC in the prospective, population-based Iowa Women's Health Study (IWHS). IWHS participants, ages 55 to 69 years, provided data regarding height; weight; weight at ages 50, 40, 30, 18 years; hip circumference; and waist circumference at baseline (1986). Derived variables included body mass index (BMI), waist-to-hip ratio (WHR), and "overweight years" (OWY; conceptually similar to cigarette pack-years). Incident CRC cases (n = 1,464) were ascertained from the State Health Registry of Iowa, through 2005. Multivariable Cox regression models were fit to estimate body size-associated CRC risks. Among 36,941 women (619,961 person-years), baseline height, weight, BMI, hip circumference, waist circumference, and WHR were all positively associated with incident CRC (Ptrend ≤ 0.003 for each). Baseline BMI yielded the highest CRC risk estimates (obese III versus normal, RR = 1.56; 95% CI = 1.10-2.22; Ptrend < 0.001) and was more closely associated with distal than proximal tumors (Ptrend < 0.001 versus 0.06). Conversely, height was more closely associated with proximal than distal tumors (Ptrend < 0.001 versus 0.04). Other body size parameters were less predictive of incident CRC. These data strongly support a positive association between increased body size and CRC risk among older women. Further investigation of when increased body size has the greatest effect on CRC risk (i.e., early adulthood versus later adulthood) might also be informative, particularly with respect to defining subsite-specific pathways of colorectal carcinogenesis.