Based on the propane pre-cooling and dual-nitrogen-expansion sub-cooling (C3-DN2) process used for floating liquefied natural gas production, storage and offloading unit (LNG-FPSO), a pilot unit will be built to test the rationality of process configuration and key control modes, and to study the operability and flexibility of its application in offshore cases. In order to define the capacity, the pilot unit should keep the consistency of process configurations and key control modes, and the similarity of moving and flow regime with LNG-FPSO. The factors affecting the capacity of the pilot unit are further investigated, and the results show that the type of key equipment to keep the consistency of key control mode greatly impacts the capacity, and the other factors have less influence. Among the main devices, the refrigerant compressor should be considered the most carefully when defining the capacity of the pilot unit, and others such as cold box, driver, turbine expander and the heat exchanger for C3 are less important. Therefore, it is better to select the same type of refrigerant compressor in the pilot as that in the LNG-FPSO, and to design two plate-fin heat exchangers in parallel in the cold box.