<p>Understanding how resource quality influences the life-history traits of organisms has been a pivotal question in ecological and evolutionary studies, especially for insects, which usually present short generation times and are easily reared in the laboratory. Using the study system composed by the host plant <i>Leucaena leucocephala</i> and the seed-feeding beetle <i>Acanthoscelides macrophthalmus</i>, we investigated which of the following seed traits, biomass (i.e., seed size), water content, <i>C</i>/<i>N</i> ratio, hardness, and phenolic contents, influenced the body size and the sexual size dimorphism (SSD) of this beetle. The effect of seed quality was investigated over two generations, excluding larval competition. We also evaluated female fecundity over one generation. For both generations, we found that female body size was not affected by any of the seed traits investigated. However, for the males of the <i>F</i>₁ generation, the pronotum width was positively related to seed biomass and inversely related to the <i>C</i>/<i>N</i> ratio, and the elytron width and length were both positively related to biomass and hardness, whereas in the <i>F</i>₂ generation the three morphological traits were all positively related to hardness and inversely related to water content. We also found that the number of eggs laid decreased linearly with hardness. The SSD was significant only for the <i>F</i>₁ generation according to changes in seed biomass and hardness. In summary, we found that biomass and hardness were the most important traits that affected body size variation and SSD when larval competition was absent, even though this effect was observed only for the males.</p>