This is a great question and an important one when deciding on a school for your child. Research has indicated that smaller class sizes do positively impact students. The following is a reference article listing the benefits of smaller classes and at what age does class size affect students.
There is a lot of compelling evidence that suggests that reducing class sizes leads to greater student achievement. The biggest obstacle to implementing these findings is a lack of resources. Smaller classess means more trained teachers, more classroom space and more equipment.
Here is some information on the subject from the Department of Education and the American Federation of Teachers. Hope this helps.