I remember having a discussion about class sizes and someone brought up an answer from the Gemara (I think Kidushin, but am not sure) where it states exactly how many pupils may be in a class with how many teachers and/or aids. Could you direct me to where this can be found in the Gemara?

The source is in Baba Batra 21:1. It was said by Rava. The Rambam brings it down in Hilchot Talmud Torah 2:5.