This paper assessed the relationship between farm size and productivity performance in Brazilian Amazon. We built two productivity indicators: technical efficiency and land use efficiency. We used Stochastic Frontier Analysis and 2006 agricultural census data to derive the efficiency measures and to assess their relationship with farm size. Results pointed out that the average Amazonian farm is productivity inefficient. The average farm could increase its agricultural production in 35.3% using the current amount of inputs. For land use efficiency, results indicate that farmers could reduce agricultural land in 90% and produce the same output using the current amount of labor and capital. Our measures of productivity presented a nonlinear relationship with farm size. However, both relations possess a similar turning point around 16,500 hectares. For policy analyses purposes, the actual relationship between farm size and productive efficiency is negative, as the turning points are far above the average farm-size in the region. Acknowledgement : This study is a part of first author's dissertation which was funded by a Capes/Embrapa doctoral scholarship. This research is supported by a CNPq research grant.