Functional size measures are used in several software development processes, because they can be applied as soon as the functional user requirements of a software application have been specified. Quite often, functional user requirements are used as the main input of effort and cost estimation. Since effort and cost estimates are fundamental for decision making, project managers would like to get reasonably accurate estimates even before user requirements have been specified completely and at the proper level of detail. To this end, early size estimation methods have been proposed. In this paper, we consider a specific situation; namely, we consider processes in which the requirements elicitation and modeling phases are carried out according to the ICONIX process. The ICONIX process involves a few stages, each one tackling an aspect of requirements and delivering a specific artifact, in the form of a UML model. We explore the possibility of deriving from ICONIX artifacts the information needed to estimate the final size of the application. To this end, we analyzed the requirements of 21 software applications and derived statistical models that correlate the information extracted from ICONIX artifacts to the size, expressed in function points, of the application. The analysis of the achieved estimates shows that the approach is viable, in that it provides early estimates with good accuracy. KEYWORDS early size measurement, function points, functional size measurement 1 INTRODUCTION Functional size measurement (FSM) was proposed by Albrecht 1 around 40 years ago, to provide an alternative to ''physical'' measures of software, like lines of code (LoCs). Albrecht's function point analysis (FPA) was received with great favor by practitioners, because it provides the means to get a proper measure of the size of software based on functional user requirements (FURs). Before the introduction of FPA, when an estimate of software development cost was needed at the beginning of the development process, one had to estimate the size in LoCs, which was quite difficult, since the size in LoC depends on several factors (including the experience of developers and the programming language, for instance)that may be unknown at estimation time.Function point analysis was a breakthrough, in that it provided a measure that could support cost estimation in the early phases of development, when it was most needed. The success of FPA induced the creation of the International Function Points User Group (IFPUG), which took charge of maintaining the FPA method and the official measurement manual. 2 Function points (FPs) were also recognized as a standard by International Standards Organization (ISO), 3 in their unadjusted form. Throughout this paper, we always make reference to unadjusted FP (UFP).Functional size measure requires that FURs are known completely and in detail. However, quite often, functional size measures are needed before the requirements elicitation and specification phase is complete, i.e., when FUR are still incomplete and...