Analysing, scheduling, developing, testing, and maintaining are the main activities of software project management (SPM) under software engineering. Software size is the major factor determining cost, time, and effort for developing software applications. The successful completion of all the activities of SPM requires an accurate determination of the cost, time, and effort required for developing the application. That is, the real size of the software application is the major determinant of all the SPM activities. The existing traditional application-oriented and single-dimensional software sizing techniques will not give the actual size of modern software applications like artificial intelligence (AI) and big data analytics (BDA). The imperfect estimation leads to wrong predictions about the cost, time, and effort required for software application development. The wrong estimates will affect the entire SPM activities, resulting in a delay in delivery, a loss in predicted cost, an imbalanced effort level, and the dismissal of the software development process. The major software industries are facing the same problem with wrong size estimates. The updated and dynamic new size estimation technique is highly essential for this current SPM environment. The novel approach of modern metrics (MM) addresses all the technical challenges facing software engineering for the software size estimation process. This article analyses the major issues of the software size estimation process, major functional and non-functional units associated with software size estimation, the architecture of MM, and an algorithm for software size estimation using MM.