The paper presents a prototype tool developed to dimension server applications running on Jambala, Ericsson's cluster-based server platform. Its very high availability derives predominantly from software solutions in the underlying TelORB operating system. The prototype-dimensioning tool is based on a high-level model of TelORB. It follows features of the operating system rather than a given application; therefore, it is applicable to any existing or new application developed for the platform. Thus, it can assist from the beginning application designers and site engineers in tuning the system for the best performance. According to the benchmark results, with the prototype tool, we achieved up to 95% precision for the simulated application. The prototype also demonstrated a significant flexibility facilitating quick evaluation of "what-if" scenarios. Although the presented model follows the TelORB design, we believe, it is easily adaptable to other computer clusters.