The rational design of Metal-Organic Frameworks (MOFs) and massive supramolecular assemblies frequently encounters unexplained empirical limits regarding maximum coherent domain size and spontaneous lattice degradation. While localized binding energies are well understood, the macroscopic stability of these crystalline networks often relies on heuristic thermodynamic approximations. This paper introduces a deterministic, scale-invariant stability threshold for supramolecular assembly. By analyzing the continuous spatial competition between local restorative binding affinity and the propagation of conformational strain across the network, we derive a rigid geometric boundary (λc=γ). We demonstrate that macroscopic lattice stability is not merely a function of thermal equilibria, but a strict structural threshold, providing a predictive parameter limit for synthetic materials chemistry.