Cimbicidae is a small family of Tenthredinoidea, containing the largest extant true sawflies. Body length variation across the family is mapped on an existing phylogeny, and some potential drivers of body size increase are explored: intrasexual competition between males, and mimicry patterns. Body length varies substantially within Cimbicidae, the largest species being found within the subfamily Cimbicinae. The smallest species occur in Corynidinae; this subfamily has experienced an overall decrease in body size. Initially, body size increase in Cimbicinae is correlated with males becoming larger than females, increase in relative mandible size in both sexes but especially in males, and increased hind femur width/length ratio in males only. Relative mandible size decreases in Cimbex spp. + Odontocimbex and hind femur width/length in male Leptocimbex spp., respectively. The record sizes obtained in Cimbex spp. + Odontocimbex are the result of a further increase in body size, coupled with increase in hind femur width/length ratio in males. Black and yellow body colouration and other potential mimicry patterns are observed in a number of genera of Cimbicinae, but are probably not ground plan features of the subfamily. Outside Cimbicinae, black and yellow body colouration is most frequent in Corynidinae, but it cannot be decided if it is a ground plan trait of this subfamily. Intrasexual competition seems to be more likely to have influenced size increase in Cimbicinae than adaptations to mimicry.