COSMIC functional size measurement method can be used in all the business process life cycle phases to early estimate the business process effort. It measures the functional size of BPMN models, at high granularity level, by considering functional processes that move data or manipulate data. However, COSMIC neglects business process control structures (XOR pattern, choice pattern, Loop, etc.). To measure the behavioral size of a BPMN model, this paper explores BPMN sizing at a fine granularity level by taking into account the behavioral aspect expressed by the control structures in the design phase. The behavioral size of BPMN model is defined based on the mapping between BPMN control structures and the corresponding flow graph aspects. For this purpose, we refer to the process recommended in Software Metrics and Software Metrology, and we propose to design a measurement method at a fine granularity level. The proposed method aims to improve the measurement of BPMN model size by taking into account the data manipulations represented by the behavior of the business process model. Consequently, it will be feasible to measure the size of functional requirements in terms of COSMIC Function Point (CFP) units and to measure the size of behavioral aspect in terms of Control Behavioral Manipulation (CBM) units. These measures (CFP and CBM) represent complementary aspects of business process size and can be used as distinct independent variables to improve effort estimation models.