In recent years, the estimation of effort, budget and time has been a fundamental step in the software project life cycle. Having this knowledge it is possible to manage and control a software project. So that, it is important to have these estimations as early as possible in order to produce a sound schedule for the project. Taking into account that BPMN diagrams are widely used in industry for analysis, in this paper we present a functional size measurement procedure based in the COSMIC method that measure these diagrams. With this size it is possible to obtain estimations at early stages of software cycle.