The Ward-based Approach (WBA), which has been introduced in the municipal solid waste management of Dhaka North and South cities and Chattogram city in Bangladesh, is a decentralized waste management system in the wards, the smallest administrative unit in the city area. In this paper, the optimal population size is examined for decentralized waste management in terms of the cleaning services that are the characteristics of waste management in Bangladesh. In addition, although it is common to set the population size that minimizes the expenditure per citizen as the "optimal population size", since basic data is not sufficient in Bangladesh, regression analysis was performed by setting the number of cleaning staff in the cleaning business as the objective variable. As a result, it was found that the optimal population size is about 94,460 (between about 84,135 and about 104,745 in the 95% confidence interval). This suggests that in the case of Bangladesh, a “demerit of scale” could influence on a smaller scale than expected in Japan.