In which stage are team size and composition defined?

The stage where team size and composition are defined is the forming stage. During this initial phase, team members come together to understand the objectives of the group and start to establish their roles within the team. Here, the focus is on introducing members, outlining the team's goals, and determining how many members will be involved. This is critical as it sets the foundation for team dynamics and helps in establishing the structure necessary for effective collaboration later on.

In contrast, the other stages focus on different aspects of team development. The norming stage is characterized by the establishment of norms and relationships within the team, the storming stage involves conflict and competition as members begin to assert themselves, and the performing stage is when the team is fully functional and works towards achieving high-level performance. Thus, the defining of team size and composition clearly aligns with the forming stage, making it the correct answer.