In many animals, body size plays a crucial role in mating success in the context of competition and preference for mates. Increasing evidence has shown that male mate preference can be size-dependent and, therefore, an important driver of size-assortative mating. To test this theory, mate choice experiments were performed during the three consecutive stages of mating behaviour, namely trail following, shell mounting and copulation, in the dioecious mangrove snail, Littoraria ardouiniana. These experiments identified two possible forms of size-dependent male mate preference which could contribute to the formation of size-assortative mating in these snails. Firstly, whereas small males were unselective, large males were selective and preferred to follow mucus trails laid by large females. Alternatively, the results can also be interpreted as all males were selective and adopted a mating strategy of selecting females similar to, or larger than, their own sizes. Both small and large males also copulated for longer with large than with small females, and this was more pronounced in large males. When two males encountered a female, they engaged in physical aggression, with the larger male excluding the smaller male from copulating with the female. This study, therefore, demonstrated that size-dependent male mate preference may, along with male–male competition, play an important role in driving size-assortative mating in these mangrove snails, and this may also be the case in other species that exhibit male mate choice.