I'd say the most important characteristic is whether the team can work together effectively.

As an example, one could consider the various people working on an individual open source project as part of a team. And most successful O/S projects rarely require everyone to meet together at the same time.

I personally strongly prefer small teams (I find even 8 members too large), but arguably certain projects may need bigger teams, and I think they should still be able to practice Agile with the bigger team, which scrum doesn't allow for.