Early software size estimation is essential for good project management. Although several proposals to estimate software size from requirement specifications exist, most of them have not been properly defined or automated. This paper presents the design and automation of a measurement procedure (ReqPoints) to estimate the size of object-oriented software projects from a requirements specification. The procedure is based on a requirements engineering approach that provides a MDA framework for requirements specification and model transformations to obtain the architecture of UML models. Specifically, a set of measurement rules is defined as a mapping between the concepts of the Requirements Metamodel onto the concepts of the Function Point Analysis (FPA) Metamodel. A Requirements Estimation Tool (REST) was built to automate the measurement process. We demonstrate the feasibility of applying the estimation tool to a case study.