There's not really a minimum size involved. I'd say it's more of a matter of logistics. Can you place the speakers correctly so as to maximize the spatial effects of these channels in your listening position?
i.e. my listening room is about 12' by 16' and I cannot, by any stretch of the imagination, place any more than the two rear surrounds and get any sort of decent effect. IOW, anything more than 5.1 would be a waste the way my room is laid out, and with walls, openings and such, my choices are limited.