This paper discusses the impact of size on software development in relation to other major cost and effort drivers, and describes current methodologies for estimating size. Automation, through computer hardware and software, has enabled great improvements in training, by making training more efficient and more effective. Computer-based training packages have shown great potential for cost savings as well as improvements in the quality and consistency of training delivery. And embedded training routines in weapons and tactical information systems allows trainees to conduct realistic exercises on the same hardware that they will use in combat. Estimating the cost of training software development is critical to developing specifications and making informed decisions about program development. Although many factors influence software development, software program size is one of the key determinants of cost and effort. This paper discusses personnel capabilities, development resources and specification characteristics, and the relative impact of size on cost & effort is demonstrated. The paper also describes commonly used software size measures. Size metrics discussed include source lines of code (SLOC) as well as functionality measures (function point analysis). It also discusses methods and demonstrates tools for developing accurate estimates of software size. Various size estimation methods are briefly described, and the SEER-SSM software sizing tool is demonstrated and discussed in detail. SEER-SSM uses a relative comparison methodology with reference programs of known size to develop accurate, probabilistic estimates of software size.