One of the significant challenges of Functional Size Measurement (FSM) is the underlying variations among FSM methods. A software product has different functional sizes in different units of measurement when measured by different methods. On the other hand, FSM methods are based on similar concepts and use comparable attributes in their measurement processes. In this paper the conceptual similarities and differences between FSM methods are discussed and a model for unification is introduced. The model is evaluated by means of case studies for measuring the functional size of software systems simultaneously by using IFPUG FPA, MkII FPA and COSMIC FFP methods.