The purpose of this thesis is to explore the relationship between team size (number of team members) and team performance in management teams. There is a lack of empirical research exploring the potential links between these two elements within management teams. Further, little attention has been paid to potential mechanisms affecting this relationship. In this study, team psychological safety has been examined as a potential mediator in the size-performance relationship, hypothesizing that team size and team performance are negatively related and that team psychological safety will mediate the relationship between team size and team performance. Data was collected using a questionnaire titled “effect”, which measured effectiveness within 215 management teams in Norway and Denmark. Results showed that there was a negative association between team size and team performance and that team psychological safety significantly mediated this relationship. These findings contribute to the understanding of the relationship between input factors, processes and outcomes in management teams, showing that team size negatively affects team performance. Furthermore, larger teams tend to be associated with lower levels of team psychological safety, which in turn is negatively related to team performance.