Recent literature has focused on how school size influences both costs and outcomes (for example, test scores and educational attainment). It also emphasizes how school size may change the nature of educationally relevant social interactions among students, teachers, and administrators. However, there appears to be little corresponding evidence on how school size influences patterns of parental involvement in schools. In this study, the authors present new empirical evidence on whether the size of public high schools influences measures of parental involvement and social capital. This analysis is based on nationally representative data from the base year of the Education Longitudinal Study of 2002 (ELS:2002). In the absence of a compelling experiment, the authors adopt an approach developed in a recent study by Joseph G. Altonji, Todd E. Elder, and Christopher R. Taber on the effects of Catholic schools. Following their lead, the authors attempt to establish bounds on the causal effects of school size by using the differences in observed traits across parents connected to smaller and larger schools as a guide to the size and direction of their potentially confounding unobserved traits. Then, they provide brief discussions of the school-size literature and the possible relationships between school size and the engagement of parents. This is followed by a discussion of the ELS:2002 data, a presentation of their baseline, multivariate analysis of these data, and a presentation of the results of their bounding exercise. The results of this study provide tentative evidence that small schools are more effective in promoting parental involvement in schools as well as engagement with the broader community. Specifically, the authors find that in rural communities smaller high schools not only increase the probability that parents take part in PTA activities and volunteer at the school, but also promote some measures of social capital. (Contains 6 tables and 27 notes.)