The RCM domains must cover most of North America to satisfy needs of associated climate change impacts programs, raising questions about the influence of domain size on results. NARCCAP RCM modelers are thus conducting a systematic sequence of domain-size tests with each of their participating models. The collective testing also provides a prototype framework for NARCCAP's overall simulation and assessment program. Tests include several models using NCEP/DOE Reanalysis 2 to simulate 1979 for domains covering most of North America or larger. We assess domain sensitivity and its significance through comparisons with several observed fields (e.g., temperature, precipitation, circulation). These tests form an important foundation for estimating uncertainty in regional climate change scenarios.