This work examines whether a physical device embedded in a parent universe can perform a strong and cosmologically complete simulation of an observable universe comparable to ours. Under three extremely general physical assumptions—finite signal propagation, attractive gravity with gravitational collapse, and non-zero cosmic expansion—we derive four independent constraints on the size of any possible simulator. These constraints arise from information-capacity bounds, gravitational stability, causal-update requirements, and Hubble-scale coherence. By varying all relevant physical parameters over the full stability range of coherent universes (±3 orders of magnitude), we show that the lower bounds (information and gravitational) are always larger than the upper bounds (causal-update and horizon). This makes strong and cosmologically complete simulations physically impossible in any universe capable of hosting complex structures and observers. This result complements previous computational, epistemic, and astrophysical analyses by demonstrating an explicit physical incompatibility between the required constraints for universe-scale strong simulation.