This is the dataset of the paper "The relative importance of body size and UV coloration in influencing male-male competition in a Lacertid lizard" published in Behavioral Ecology and Sociobiology by Names et al. (2019). It includes a metadata statement and five data spreadsheets. Abstract of the paper Communication via color signals is common in natural systems. Ultraviolet (UV)-blue patches located on the outer-ventral scales of some Lacertid lizards are thought to be involved in male-male competition. However, the mechanisms that maintain their honesty remain unknown. Here, we use the common wall lizard Podarcis muralis to test whether the lateral UV-blue spots are conventional signals, the honesty of which is guaranteed by receiver-dependent costs, and discuss their potential role as an amplifier of body size. We first described the morphology and reflectance properties of lateral blue spots in common wall lizards and investigated how they influence male- male competition. Spot size and number, UV chroma, and conspicuousness (calculated using vision models) were significantly greater in adult males relative to adult females and adult males relative to juveniles. Total spot area (and not spot number) of adult males was positively correlated with body size. We conducted staged competition encounters between focal males and smaller or larger rivals with control or manipulated spots. Spots were enlarged in small rivals and reduced in large rivals to disrupt the phenotypic correlation between spot area and body size. Aggressiveness and dominance were positively influenced by body size in control encounters. Spot manipulations resulted in greater submission and less aggressiveness in focal males. These results contradict the predictions associated with conventional signals and amplifiers, but suggest that spots contributed to opponent evaluation during short-distance encounters between competing males.