We investigated the effects of assessor team size on the accuracy of ratings in a presentation exercise as it is commonly used in assessment centers and compared it to the effects of two factors related to assessor expertise (assessor training and assessor background). On the basis of actual ratings from a simulated selection setting (N = 383), we sampled assessor teams of different sizes and with different expertise and determined the accuracy of their ratings in the presentation exercise. Of the three factors, assessor training had the strongest effect on rating accuracy. Furthermore, in most conditions, using larger assessor teams also led to more accurate ratings. In addition, the use of larger assessor teams compensated for having not attended an assessor training only when the assessors had a psychological background. Concerning assessor background, we did not find a significant main effect. Practical implications and directions for future research are discussed.