This study reports a measurement support system, which was developed based on the XForms-Format user interface (UI) specifications in order to facilitate the process of measuring the functional size of interactive software. XForms is the next generation of Web Forms. As it is capable of describing the purpose and the presentation of the Web form separately, the semantics of user-computer interactions, which is essential to the functional size measurement (FSM), can be extracted from the specification. In this study, we propose several extensions to the current W3C?s XForms for its application in FSM. The procedure of measuring the functional size, which was proposed in our past study, is further automated based on the XForms-Format UI specifications. The interactive part of a car-navigation software product was measured with the support system and the results suggest that this system is effective in generating reliable results and reducing measurement costs.