Bottom trawls with large mesh sizes in the forefront section are commonly used to target the largehead hairtail (Trichiurus lepturus), which is one of the most ecologically and economically important fish species in marine capture fisheries in China. In this study, the size selectivity of four diamond-mesh codends with mesh sizes of 30, 35, 40, and 45 mm was tested using the covered codend method in pair trawls with large mesh sizes at the forefront section of the South China Sea. The experimental codends were termed D30, D35, D40, and D45 according to their mesh sizes. Catch data were analyzed using the double-bootstrapping technique to account for uncertainties from both within- and between-haul variations, compare the size selectivity of tested codends, and estimate the exploitation pattern indicators for largehead hairtail. The results demonstrated that by increasing the mesh sizes of the codends, the 50% retention length (L50) of the largehead hairtail increased, whereas the capture probability and exploitation pattern indicators for undersized individuals decreased. For instance, the L50 was 12.22 cm for the D30 codend, and increased to 13.24 cm for the D45 codend. The retention proportion of undersized individuals was 57.10% for the D30 codend and decreased to 40.06% for the D45 codend. The differences in the L50 and exploitation pattern indicators between the tested codends were not statistically significant. Differences in the retention probability of undersized individuals in some length ranges between the codends were statistically significant. Considering that the mesh size used in trawling fisheries is usually small, often close to 30 mm, our results demonstrated that increasing the mesh size from 30 to 40 or 45 mm would improve size selectivity for the target species. The L50 values increased, whereas the retention probability of the undersized largehead hairtail decreased. However, the size selectivity of the codend with a mesh size of 40 mm (D40), currently regulated by the government, was poor for the target species under the management regulation scenarios of the minimum landing size (23.0 cm). Consequently, over 93% of the largehead hairtail were undersized and thus discarded. This finding suggests that more selectivity studies are needed to further improve the selective properties and achieve sustainable development of the bottom trawl fishery. Additionally, research on the minimum landing size for largehead hairtails is warranted to supplement the minimum mesh regulations in the South China Sea.