Body size is one of the most important life-history traits of insects. As demonstrated in numerous studies, insect body size can have substantial effects on a number of biological characters, which in turn may influence population development as well as community structure and functions. In this paper, based on the studies published in the past two decades, we reviewed the major findings in this field, and focused on the effects of body size on following traits:1) courtship, mating, fecundity of females and males, and fitness of their offspring; 2) flight performance, and the relevant behaviors such as foraging, aerial courting and mating; 3) food-uptake capacity and the type of preferred diets; 4) competition and defense capacity; 5) resistance to environment stress; and 6) labor division in social insects. In most cases, larger individuals are superior to small ones in certain aspects, reproduction, flight and adaptation to adverse environments in particular, which subsequently favors the increase of population fitness. We presented some viewpoints as to future studies, such as taking more biological traits into consideration when evaluating associations between body size and insect reproduction, and paying more attention on the ecological results of body size variance at the community level. From the practical perspective, it is suggested to take body size into account when developing insect-pest control or beneficial-insect utilization strategies.