What will determine the size of a class?

While teaching English, I always wonder what the size of each class matters. For example, I'm having a problematic class of 18 people. The reason I'm messed up with the class is that most students are so immature I need to give instruction directly to each student. If the class contained less than 10 people, I could handle it.

If a student needs individual instruction, I can't handle other students at that time.

So, basically, I agree with the opinion that the class should be as small as it can be. However, my dad always tells me it is a stupid idea. According to his experience, if class is too small, teachers would have to worry about each student too much. So, he says it is suppoesd to be around 40.

Which do you prefer, a small class or a big class?

I don't agree with him, but his words helped me wonder which side should be listened the most when it comes to determind the class size. Teachers (or managers of schools) or students? Actually, it isn't an easy question. So far, I have been thinking that it should be determined by students' opinions, but it is safe to say that teachers' opinions are also need to be considered to some extent. Again, I belive that small clasees are more effective as long as I refer to the effect of language learning.