Software size is a fundamental product measure that can be used for assessment, prediction and improvement purposes. However, existing software size measures, such as function points, do not address the underlying problem complexity of software systems adequately. This can result in disproportional measures of software size for different types of systems. We propose a vector size measure (VSM) that incorporates both functionality and problem complexity in a balanced and orthogonal manner. The VSM is used as the input to a vector prediction model (VPM) which can be used to estimate development effort early in the software life-cycle. We theoretically validate the approach against a formal framework. We also empirically validate the approach with a pilot study. The results indicate that the approach provides a mechanism to measure the size of software systems, classify software systems, and estimate development effort early in the software life-cycle to within /spl plusmn/20% across a range of application types.