‘Resource dilution’ has been invoked as a possible mechanism to explain the inverse relation between sibship size and sibling heights in European populations (Öberg, 2015). Alternative explanations include confounding of the relation by other measured or unmeasured family characteristics including socio-economic position or birth order. It is difficult to quantify the contribution of any factor in isolation. To examine the question, we accessed data from the national birth cohort of 389,287 Dutch conscripts born in 1944–1947 and examined for military service at age 18. The records include all men of Dutch nationality born between January 1, 1944 and December 31, 1947 examined for military service in the Netherlands. The birth cohorts provide a well-defined study population to reliably assess the impact of family size and birth order on adult height, accounting for potential confounders. The cohorts include a large number of high-parity families, provide reliable information on both family size and birth order, and differentiate between all birth orders. The military examinations provide reliable information on height for all study subjects and uniform measurements at age 18. We show that recruits from larger families are shorter than recruits from smaller families; that birth order effects are small in relation to family size effects; and that birth order and family size effects are comparable in recruits from higher to lower socio-economic backgrounds. Recruits from higher backgrounds are significantly taller however. Our findings are compatible with a ’resource dilution’ hypothesis, but our data provide no information on any specific differences in the childhood environment in terms of nutrition, education, or other family behaviours could explain the observed height differences at age 18. Additional studies will be needed to identify what differences in specific family behaviours could lead to changes in height development.