Size related measures have traditionally been the basis for effort estimation models to predict costs of software activities along the entire software product life cycle. Object-Oriented (OO) systems are developed and evolve by adding/removing new classes and modifying existing entities.We propose an approach to predict the size of changes of evolving OO systems based on the analysis of the classes impacted by a change request. Our approach can be used both in iterative development processes or during software maintenance.A first empirical evaluation of the proposed approach has been obtained by applying our tools to the post-release evolution of OO software systems available on the net. The systems were analyzed, and models to predict added/modified LOCs from added/modified classes were statistically validated. In the paper preliminary results of the above outlined evaluation is presented.