Given that ORS is a former teacher, rather than engaging in belittlement for the pride of who "wins the skirmish", wouldn't it be more constructive to ask her opinion of what worked best for her in class size during her career?
Then, offer your summary of what research shows as optimal, and invite her comment? (you could even do that in different order)
We know here in WA state there is effort to reach 17.3 kids per class, and one would think that is the result of research, with opinion of experienced teachers considered.
I know that 18 is an optimal size, less than that and it actually becomes more difficult! More than that not a problem at all, to a point (25-30?). Has to do with division of lesson plans, group dynamics, age of kids taught, and, number of "challenging" kids above and below grade expectations.
But, WA state's "17.3" is a concept, not an end, an average number to move to (and yes, I added the .3 for effect).
It also depends on the grade and subject(s) taught.