To estimate software code size early in the development process is important for developing cost-efficient embedded systems. We have applied the COSMIC Functional Size Measurement (FSM) method for size estimation of embedded software components in the automotive industry. Correlational studies were conducted using data from two automotive companies. The studies show strong correlation between functional size and software code size, which is important for obtaining accurate estimation results. This paper presents the characteristics and results of our work, and aims to provide a practical framework for how to use COSMIC FSM for size estimation purposes. We investigate the results from our earlier correlational studies, and conduct further studies to identify such a framework. Based on these activities, we conclude that a clear purpose of the estimation process, a well-defined domain allowing categorization of software, consistent content and quality of requirements, and historical data from implemented software are key factors for size estimation of embedded software components.