Quantum engineering seeks to exploit quantum information to build, among others, computing, cybersecurity, and metrology technologies. Quantum Software Engineering (QSE) focuses on the information processing side of these technologies. Historically, quantum (software) engineering has focused on development in controlled research environments and 'in the small'. As the field progresses, we should expect to see more large-scale quantum systems to be deployed as 'real-world' products and services. An essential tool in (classical) software engineering and development has been software size metrics. Calculating/estimating the size of a piece of software, to be developed or pre-existing, is an essential step in its engineering. Quantum software will be no different. Here we introduce Q-COSMIC, a technique for measuring the functional size of quantum software, based on the well-regarded COSMIC standard (ISO/IEC19761) for classical software