To develop an evolutionary theory of social decision making, we require an understanding of how individuals utilize environmental cues to form decision rules. We exposed ‘cleaner’ fish (bluestreak cleaner wrasse, Labroides dimidiatus) to a biological market task, where giving priority to an ephemeral (i.e. ‘visitor’ client) food plate, over a permanent (i.e. ‘resident’ client) plate, doubled the food reward. Previously published experiments revealed that adult cleaners from a complex social environment regularly solved this task and outperformed adult cleaners from a simple social environment as well as juveniles from both habitat types. In these studies, plates were differentiated by colour and/or colour pattern. However, client size is another potentially useful cue that may be used by cleaners to solve the biological market task in nature, as visitor clients are typically larger than resident clients. Here, we tested cleaners in a setting where plates differed only in size and not colour/pattern: the majority of cleaners exhibited a spontaneous preference for inspecting larger plates or were more likely to reach the task-solving criterion if the visitor plate was larger. All cleaners were able to solve the task when we incorporated both size and colour/pattern cues; however, only cleaners from the complex social environment settled on the more precise colour/pattern cue. In contrast, cleaners from the simple social environment relied on size as the primary, yet less precise, cue to solve the task. In conclusion, our results strongly suggest that intraspecific variation in the performance of cleaners in the biological market task is based on variation in the relative salience of available cues and correlates with variation in a cleaner's natural social environment. Variation in the relative salience of available cues may therefore explain a portion of the intra- and interspecific variance in cognitive performance and social behaviour documented in other animal species.