Existing computational models differ in predictions concerning the set size effect in the Stroop task, which is the relation between a number of stimuli/responses and the magnitude of interference. In some studies, no set size effects were observed, while in other substantial set size effects were present. We administered two experiments in order to resolve this discrepancy in the case of manual Stroop task. Experiment 1 compared conditions including four, six, and eight stimuli/response mappings in the picture-word Stroop. No reliable set size effects have been found, apart from a weak effect observed when working memory load imposed by the task was deliberately decreased. Experiment 2 tested conditions consisting of four versus eight mappings in the color-word Stroop, and it has replicated results of Experiment 1. As both experiments had sufficient power to detect set size effects if they existed, our data are inconsistent with models which predict such effects.