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. According to this line, many countries in the world are trying to downsize the schools for better education. In this study, the average statistics on school size in USA and Korea are compared. Some specific cases, for example Texas and North Carolina for USA, and Incheon Metropolitan City for Korea, are analyzed concretely in order to extract an idea of optimum school size. The average school size in USA is around 700 students in a school located in big cities. And many academic researches suggest that the most effective school size is 600-900 students in a school. But in Korea, most schools in metropolitan cities have more than 1,000 students. Taken together and considering our current situations, this research suggests three concepts on school size: ideal size, optimum size, maximum size. And also suggests 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 school size.