Animal communication depends on signals conveying information to a receiver who must perceive and decode them. Signals involved in territoriality are usually complex stimuli that should be correctly interpreted to avoid unnecessary conflicts. Lacertids use both visual and chemical stimuli in modulating their aggressive response against conspecifics and the rival’s size is one of the most important information, affecting the success probability in a combat. To assess the actual ability of decoding information about rival’s size based on its chemical stimulus alone, 60 males of Podarcis muralis were tested for three consecutive days in an arena bearing a mirror (to simulate an equal-sized intruder), and the chemical cues (femoral secretions) from an unknown individual of different size. Significant differences were observed in tongue-flicks number, which grew as the size difference between the focal lizard and the secretion donor decreased. This can be interpreted as the need for the lizard to better evaluate the potential competitor’s characteristics. The size difference also affected the number of bites against the mirror, which increased when the size of the focal lizard was larger than the donor triggering the aggressive response with a higher probability of winning the contest, as expected if the focal lizard had correctly decoded the information about the opponent’s size by chemical stimulus. Although previous studies have shown that some components of the chemical signals are potentially informative about the signaller’s size, this is the first demonstration that male P. muralis are actually able to decode and use such information.