Personally, I think the features should determine this. If a piece of work is huge and needs many team members, then no reason to split the team off into pieces.

Even in a big team, a BA, dev and tester should be collaborating during ownership of the user stories and therefore, the team should still tick over nicely.

If you get two seperate features on the go though, that is a good point to split the team and have feature teams.

So I'd say it is the work being done that would decide the teams as opposed to just splitting when a team becomes a certain size.