Recent evidence indicates that the UML (Unified Modeling Language) is the most preferred and widely used modeling technique for object-oriented analysis and design. With UML becoming so popular, there is a need to have good quality assurance techniques for projects using it. Our focus in this study is on the inspections of UML design documents. The basic premise of software inspections is that they detect and remove defects before they propagate to subsequent development phases, where their detection and correction costs escalate. However, the performance of inspections can vary considerably, making it important to optimize inspections. One approach for optimizing inspections is by controlling the inspection team size. This paper presents an empirical evaluation of the optimal team size for UML design inspections. Our results show that there is no single optimal team size. Optimal team size in fact depends on various conditions such as the cost of defect detection late in the process, and inspection meeting duration. This paper quantifies these factors and proposes optimal team sizes under various conditions. Our results also indicate strongly that contemporary suggestions of only two-person inspection teams are far from optimal.