Adult size is the trait most closely correlated with reproductive output in insects, but may also have important selective implications determining additional fitness gains. In longhorn beetles, adult size-mediated ultimate benefits may arise from mate choice, male antennal spread width or male fighting for mates. In this paper, we examined factors potentially shaping adult size of Cerambyx welensii Küster (Coleoptera: Cerambycidae), an emergent oak (Quercus Linnaeus; Fagaceae) pest. Sex ratio and adult length/weight allometric relationships were also explored. Overwintering adults were collected inside oaks in Extremadura (southwestern Spain) during 2011–2017 to ensure that larval development was completed in the wild. Sex, host species, and wood quality affected adult size, though some interactions occurred and among-host differences were weak. Adults collected inside older trees and wider branches were significantly larger. Adult size was unaffected by either elevation, aspect, or population density. There was a robust allometric scaling in both sexes between elytral/adult length and weight, females being heavier than males and males longer than females when adults were large. Female-biased sex ratios occurred in old/veteran trees and in dense/crowded larval populations. We discuss these results from an evolutionary perspective considering the potential impact of C. welensii adult size on population dynamics and management tactics.