Insights on Quantum Software Functional Size Measurement: Key Concepts, Challenges and Motivations

The rapid evolution of quantum computing has led to growing interest in the development of systematic approaches to assess and manage quantum software. Among these, functional size measurement (FSM) offers a promising pathway for establishing metrics that can support project estimation, benchmarking, and quality assessment. This paper provides an overview of emerging efforts in quantum software functional size measurement with a focus on key concepts, challenges, and motivations. First, we examine the foundational principles of quantum computing in relation to the existing FSM methods, identifying both compatibilities and unique constraints. Building on this, we discuss the challenges that arise when applying FSM to quantum software, including the hardware-coupled nature of functionality, evolving semantics of data and measurement abstractions, gaps in tooling and standardization, and evolving role of FSM across the Noisy Intermediate-Scale Quantum (NISQ) and future faulttolerant eras. We then outline community-driven initiatives, such as the COSMIC Quantum Software Taskforce and the Fall 2024 Workshop, which highlight the increasing demand for structured measurement practices. By articulating these issues, this paper aims to present initial research efforts for quantum software functional size measurement and stimulate further exploration of measurement approaches that are both theoretically grounded and practically applicable in quantum software engineering.