Although Functional Size Measurement (FSM) methods have become widely used by the software organizations, the functional size based effort estimation still needs further investigation. Most of the studies on effort estimation consider total functional size of the software as the primary input to estimation models and they mostly focus on identifying the project parameters which might have a significant effect on the size-effort relationship. This study brings suggestions on how to use COSMIC functional size as an input for effort estimation models and explores whether the productivity values for developing different functionality types deviate significantly from a total average productivity value computed from total functional size and effort figures. The results obtained after conducting a multiple case study in which COSMIC method was used for size measurement are discussed as well.