One of the reasons for the success of the agricultural sector in the developed countries is regulate the size of the farms, because it helps to scale economy of production. Considering that the size of agricultural production units should be specifically determined for each product and in each area, the aim of this study is to determine the optimum size of rice farms in the Fereydounkenar county, Mazandaran province. For this study, 198 farmers were randomly interviewed in 2014. The results of the study showed that the optimal economic size of the farms in the study area was 1.949 hectares, which is 43% larger than the average cultivation in the region. The results of technical efficiency using Stochastic Nonparametric Envelopment of Data model (StoNED) also showed that the size of rice farms has a direct correlation with the efficiency of these units in the studied area and production in larger farms is more economical and efficient. Therefore, it is suggested to be considered plans to encourage rice farmers to establish cooperatives and change the farms management from small farm owners to integrated manage.