In many animals, body size is correlated with reproductive success. Selection sometimes generates striking differences in body size between males and females (i.e., sexual size dimorphism, SSD). SSD is common in spiders (Araneae), and is typically explained by selection for larger, more fecund females, and rapidly maturing, and consequently smaller males. Within a species males and females also often vary in body size.  In the false widow spider (Steatoda grossa), females are larger than males and males trade off body size for rapid development and early maturation. Moreover, males exhibit considerable variation in body size, suggesting that under certain conditions there may be advantages to large size. Here, we tested the role of male body size on mating success under non-competitive and competitive mating conditions (i.e., male-male competition) in S. grossa. We found that body size did not influence mating success or copulation duration under non-competitive conditions, but that larger males were more successful at obtaining access to females under competitive mating conditions. Additionally, we found that total copulation duration was significantly lower when a rival male was present. Our results show a large male advantage under male-male competition, which we suggest may contribute to the high variation in male body size observed in S. grossa. We further suggest that the reduced copulation duration observed under competitive mating conditions may have potential ramifications for male and female reproductive success, and discuss how patterns of selection acting on male body size might limit the extent of SSD in this species.