Input use efficiency and productivity of different sizes of farms producing FIYV Boro in some selected areas of Brahman Baria district were investigated by employing primarily a Cobb-Douglas production function. Returns to scale and farmer's capability of producing at the least cost level were statistically tested. Farm size and productivity relationships were found to be positive. Boro production was characterized by increasing return to scale for only the medium farms. Few inputs were used in Boro production at the least cost combination level. Adequate extension services including application of right quantity of inputs at right times may help achieve efficiency in input use and improve profitability.