Argyroneta aquatica is the only spider that spends its entire life under water, and is one of the few spiders in which males are larger than females. In this paper we investigated size dependent mate choice to clarify whether intersexual selection may be partly responsible for the reversed sexual size dimorphism (SSD) in A. aquatica. We found that females that only copulated once could produce up to six viable egg sacs, although the number of offspring decreased with each egg sac produced. Males are the more active sex in mate acquisition and females prefer large males as mating partners. However, females fled more often from males larger than their own size (SSD > 1) than from relatively smaller males (SSD < 1), although small males approached females more often than large males did. We found that males of A. aquatica may cannibalize females, which to our knowledge is the first account of such reversed sexual cannibalism in spiders. The extent of SSD (m > f) determined the likelihood of females being cannibalized. Apparently, avoidance behavior of females towards the preferred, large mating partners is related to the higher risk of being cannibalized. In A. aquatica, intersexual selection may stabilize male size at an optimum instead of directionally selecting for large body size.