Software functional size (FS) prediction early in its lifecycle is vital for software project management. Such prediction requires the definition of software measures in terms of the specification and/or design language concepts. Within this context, several researchers have projected COSMIC functional size measures (FSM) onto various UML diagrams. However, these projections treated the diagrams separately despite their syntactic and semantic overlap within a model. In this paper, we present a fine-grain measurement for the UML use case diagram. Being a central diagram for models derived through the Unified Process, the use case diagram is implicitly related to all other diagrams. Thus, the detailed measurement of this diagram provides a reference measurement standard for other UML diagrams, e.g., the sequence and class diagrams.