Previous research within the additional-singleton paradigm mostly focused on the effect of colour or shape on attentional capture. In this study, we explore the effect of additional singletons which differ in size from other distractors. We demonstrate that the presence of an additional singleton whose size is uniquely different from the rest of the stimuli leads to slower reaction times when participants are in a singleton-detection mode. We find no conclusive evidence for a differential effect of three size levels relative to the other stimuli. Also, we find no conclusive evidence for an intertrial repetition effect or a habituation effect.