This review discusses body size and mass as they relate to the Orthoptera (crickets, katydids, grasshoppers) and the Phasmatodea (walkingsticks). It addresses the expression, causes and consequences of size in these insects. Topics include: methodological problems in body-size research, gravity vs surface forces, allometry and scaling, Dyar's law, ontogenetic scaling, size-invariant traits and nonallometric scaling, the influence of size on physiology, function, behavior, life history, mating, fecundity, population dynamics, ecology, and community, size-clines, Bergmann's rule, sexual size dimorphism, Rensch's rule, protandry, the environmental, genetic, and physiological control of size, the evolution of size and the influence of size on evolution. Hypotheses are presented to explain why insects remain small in comparison to other taxa.