There are various software size measurement methods that are used in various stages of a software project lifecycle. Although functional size measurement methods and lines of code measurements are widely practiced, none of these methods explicitly position themselves in problem or solution domain. This results in unreliable measurement results as abstraction levels of the measured artifacts vary greatly. Unreliable measurement results hinder usage of size data in effort estimation and benchmarking studies. Furthermore, there exists no widely accepted measurement method for solution domain concepts other than lines of code, such as software design. In this study, an approach is defined to distinguish problem and solution domains for a software project and a software size measurement methodology for solution domain is proposed based on software design sizes.