Enterprise Resource Planning (ERP) projects have special characteristics that differentiate them from other kinds of software projects. Main difference that affects effort estimation results is high reuse rates of ERP projects. This paper presents our exploratory work to establish an approach to calculate reuse reflective size of ERP projects, which could be a primary input for effort estimation. We explored the usability of COSMIC function points as the base unit and its convertibility to reuse reflective size using reuse levels. We have performed a case study on an SAP Implementation project. We were able to calculate COSMIC size and reuse levels using the documents available for the project. We discuss the results as well as challenges and opportunities in the light of the case study.