Graphene’s remarkable mechanical, thermal, and electrical properties make it essential for various applications, but nanoscale experimental testing is limited. Computational models like the Atomic Scale Finite Element Method (AFEM) provide detailed insights but are computationally intensive for larger systems. This study uses a nonlinear stick-and-spring molecular mechanics model to analyze the size effect and transition to continuum mechanics in graphene under large in-plane deformations. Simulations are performed on graphene sheets of varying sizes and defect densities to identify the threshold size beyond which mechanical properties are not size dependent, comparing results between defective and pristine sheets.