It seems to be part of human nature that we are perennially tempted by the prospect of a single and powerful answer to our problems. So it is not surprising that current interest in smaller classrooms and schools has led many people including some educators to hope that smallness has the power to improve the quality of schooling. Since the beginning of modern education system, we have been trying to expand the educational opportunities for more people in sacrificing the quality of education. As a result, big classrooms and large schools in urban areas have become the hottest problem and issue in our education. Some of the researches have tried to find optimum class size for better education, but rare about school size yet. According to the available research conducted in USA on school size and its relationship to student achievement, the large majority of studies indicates that smaller is better. There are some inconsistencies with respect to high school size and achievement, but studies of school size in general have demonstrated that smaller schools are associated with higher achievement and higher rates of participation in extracurricular activities. An analyses of state-level data from North Carolina appear to provide partial confirmation of these findings, but the association between school size and student outcomes that were found are not large. But an academic study conducted by V.E.Lee and J.B.Smith suggests that the most effective size for a high school is 600-900 students in a school. Students in smaller schools than 600 or considerably larger schools over 2,100 tend to learn less. Large schools were found to impact low socio-economic status students more negatively than other groups. Taken together, the prior researches on school size appear to show, despite the existence of some contrary findings in the literature, that smaller schools are better than larger schools with respect to students achievement. Considering our current situations, this research suggests three concepts on school size: ideal size, optimum size, maximum size, and 18-24 classrooms(630-840 students) in a school for elementary schools, 21-27 classrooms(735-945 students) for junior high schools, and 24-30 classrooms(840-1050 students) for senior high schools as optimum size.