Estimating the size of a large and complex software system is a challenging task. Early in a project's life cycle, when requirements for the system may be immature and functionality defined only at a high level, resource profiles are necessary for appropriate funding, staffing, and development of a viable project plan. Similar project historical software size data and trends provide a tool to predict software size, creating a feasible estimation approach. To confidently estimate the flight software required for the Orion Spacecraft, NASA conducted an analysis of similar space and aircraft software systems. This paper presents the results of a study that compiled data from over 400 spacecraft, aircraft, and submarine software projects, which were functionally similar in nature to the Orion spacecraft flight software. The study includes the analysis and summary of overall trends in software size, development strategies, and processes. The results indicate a well-correlated upward trend in software size over time for both crewed and uncrewed space and aircraft. This trend was used to predict the estimated completed size of the Orion flight software at approximately 2.3 million Source Lines of Code (SLOC).