Body size determines mobility and fitness of insects in various ways. Yet, especially in parasitoid species, drivers of body size are poorly understood, as they can be influenced by complex interactions between parasitoid behaviour, the environment, and their hosts. We measured the body size of 393 individuals of four parasitoid species sampled with trap nests for cavity-nesting bees and wasps in the Southern Black Forest, Germany. We related the body size of the parasitoids to the size of 15 host species and the diameters of their nests along four environmental gradients (canopy cover, structural complexity, herb cover and deadwood diameter). Host identity, nest diameter, and to a lesser extent, size differences within host species, were primary drivers of parasitoid body size, albeit parasitoid species differed in their responses. For instance, when the host Trypoxylon figulus doubled in size, Nematopodius debilis (parasitizing the host directly) increased by 37% in size, while Trichrysis cyanea (parasitizing food resources) increased by only 8%. Across host-parasitoid species combinations, there was only a weak positive relationship between the size of parasitoids and their hosts. In addition, we did not observe size variations in hosts and parasitoids across environmental gradients. Our findings highlight the primary factors influencing parasitoid size, with host identity and nest diameter emerging as influential factors within and between species, though not uniformly across all species. In contrast, the relationship between parasitoid and host size and environmental gradients were less influential. Considering the environmental variables that directly affect body size, such as microhabitat conditions and biotic interactions, may further clarify the dynamics shaping variation in parasitoid size.