Some of classroom size has to do with the teacher.
My son's second grade class (public school) has 26 kids. The teacher is incredible, and she has excellent order in the class. She seems to have a very good handle on what each kid is doing. His (private school) kindergarten class had only 10 kids in it but the teacher was awful- disorganized, poor teaching, etc. So... It does vary too based on the teacher.
I do think at some point the class size can get too big and you will likely get more discipline issues, etc.
In my son's school in every class, they use something called Surround Sound. It's where the teacher wheres a clip-on microphone and it transmits her voice through overhead speakers in the class. It makes her voice a little louder. That way, she doesn't have to shout to get the kids' attention. Studies show that reading scores, etc. go up with that. It's good if the class is too big also.
