I've always used this as my basis for thinking about team sizes: https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus... - personally I say teams are ideally 4 -/+ 2 only.
In Leading Teams[1] J. Richard Hackman says 6 max based on this research: https://www.jstor.org/stable/2786271
[1]https://books.google.com/books/about/Leading_Teams.html?id=w...