Although functional size is the primary input for effort and cost estimation formulas, the relation between functional size and development effort does not always hold in practice. Calculating the magnitude of functional similarity is perceived as a partial solution to overcome this problem. In this study we applied different approaches to identify the magnitude of functional similarity in five projects. The applicability of these approaches is evaluated by observing the impact of functional similarity on effort and code size of the projects. We also discuss the challenges, difficulties and opportunities faced during the case studies.