After saying 5-7, I want to note that I was once on a cross-functional project team for system change management that had about 15 members too. Once that larger team agreed to a structured meeting management approach (which I termed RARA in my book for Roles, Agenda, Records, and Actions) using a trained facilitator as a role model recommending various methods for idea generation, problem-solving, and decision making that team worked very well in their time together and even reduced their required meeting time. Many members of this cross-functional team were team leaders for process sub-teams which contained members from functional areas in order to efficiently get research and work done outside meetings. Having a model in the larger team helped them better lead their sub-team meetings as well.
So I’m not so much into the number as what is the right size to get the team job done. Before deciding what size the team should be and who might be the best members, decide what type of team is needed. If the team is to be a working team, meaning they do assignments, coordination, and/or implementation of a plan rather than design, then this is typically better done with small teams. Larger teams are better if their needs to be communication across different functions or departments and where diversity of ideas, different perspectives, along with sharing of a lot of information is desired. Large teams work well for idea generation, design, or change creation teams.
Also look at how the team should be governed. Will it be self-directed and therefore in charge of most of the responsibility for team success? Or will it be managed by a team leader, project manager, or another level of management? After these decisions are made, look at what the requirements will be for team membership and what additional responsibilities the team leader, manager, or sponsor may have in order to help the team succeed.