Thanks to its simple design, operational flexibility and potentially higher thermal efficiency at higher pressure ratios, the free piston expander (FPE) is gaining popularity and attention from researchers. A lot of work is expanded to implement the FPE concept in the organic Rankine cycle (ORC) for waste heat recovery. However, steady-state models that predict the efficiency and power output of FPEs under varying conditions are not available. The main objective of this work is to build a steady-state model to optimize the FPE-based, waste heat recovery cycle using a suitable working fluid. A thermodynamic analysis is carried out to match the unsteadiness of the FPE with the steady heat rejection, pressurization, and heat recovery of the ORC. Entropy before condensation and internal energy after constant volume filling is optimized, keeping the thermodynamic state of the fluid coming out of the heat exchanger fixed on the saturated vapor line. From optimized values, work output and efficiency for a specified condition (hot and cold source temperatures) are calculated. Targeted power output, maximum allowable piston velocity, and frequency are constrained by the system, from which the sizing of an FPE is derived. The sizing criteria provides a mean for the selection of the optimum working fluid. The analytical results show that the efficiency increases with the increasing expansion ratio up to a certain value, but however has a negative effect on specific power. Increasing the initial volume, before the filling of the FPE takes place, decreases both the efficiency and specific power and should be minimized for optimal operation. Optimum fluid selection is also carried out for two test cases with varying hot source temperatures and maximum piston velocity.