Continuous improvement teams play an essential role when implementing a corporate improvement programme, the success of which is significantly dependent on the organization of such teams. This paper specifically addresses the effect of team size on the performance of continuous improvement teams. We take insight into a Norwegian case study during the first two years of a lean transformation to explore if there are any indications of an optimum size for continuous improvement teams. The results suggest that there may not be a perfect size for a CI team, rather the performance outcome of different sized teams may depend on the maturity level of the lean programme. Whereas in the establishment phase teams of up to twelve employees seem to perform well, this number appears to reduce when the programme matures. This study may therefore assist practitioners in establishing the right team size dependent on the status of transformation.