Native to Asia, the harlequin ladybird (Harmonia axyridis) was introduced into Europe and North America as an aphid biocontrol agent. The species has become invasive in many countries, negatively affecting native species in the introduced areas, and its spread is characterized by rapid increases in population size. In its native range, H. axyridis is parasitized by the wasp Dinocampus coccinellae, which may control the H. axyridis population. However, D. coccinellae cannot parasitize H. axyridis in introduced areas. The resistance of H. axyridis to parasitization by D. coccinellae may be key to the rapid invasion of H. axyridis. Knowledge of the fundamental ecology of D. coccinellae is needed to learn more about the relationship between the parasitoid wasp and the ladybird species it uses as hosts. In this study, we used the hind tibia length of D. coccinellae and pronotal width of host ladybirds as body size indicators, to test for a size correlation. We found a significant positive relationship between the sizes of D. coccinellae and multiple host ladybird species including Coccinella septempunctata, Coccinella undecimpunctata, Tytthaspis sedecimpunctata, Myrrha octodecimguttata, Harmonia quadripunctata, and H. axyridis from British populations and C. septempunctata brucki, H. axyridis, and Propylea japonica from Japanese populations. These results indicate that there is a positive correlation in body size between D. coccinellae and host ladybird species.