[simplyesl]

I have taught groups of 30 kids down to 5-6 kids (age: 4-5).

I find that it's easier to get the kids interacting with themselves for a longer period of time when there are more of them.
I don't think the games I used varied that much, only how we played them.
For example, "stations/run + slap" - With a small class, everyone can play. If you try it with a bigger class you either need a lot more space or the sts have to take it in turns. I usually have 4-5 groups (roughly 5 per great). This is good because they have to listen to a) which group and b) which item of vocab.

Hope that helps!