<p>This study presents a novel comprehensive investigation of functionally graded (FG) piezoelectric sandwich microplates which incorporate a hexagonal honeycomb core and that rest on an elastic foundation, a theoretical formulation which is rigorously developed within isogeometric approach (IGA) framework. Unlike previous works, the proposed model integrated modified strain gradient theory (MSGT) with a four-variable refined plate theory (RPT). Compared to nonlocal or micropolar theories, the MSGT provides a more robust representation of size-dependent effects by incorporating three length scale parameters that explicitly capture both higher-order strain gradients and the resulting stiffening effects that occur at the microscale. Furthermore, the sandwich microplate incorporates two FG piezoelectric layers composed of PZT-4 and PZT-5H that are distributed according to a prescribed power law scheme, a structural configuration which thereby enables a highly multifunctional electromechanical performance. The equilibrium equations are derived using the principle of virtual work, while IGA formulation ensures the required <i>C</i>²-continuity by employing cubic NURBS basis functions that are capable of providing at least third-order derivatives. Comprehensive numerical studies are conducted to benchmark the proposed formulation against existing analytical and numerical results, a rigorous validation process which confirms its exceptional accuracy and superior predictive capability. Beyond this fundamental validation, numerical investigations deliver new insights into the influence of length scale parameters (LSPs), power index and external electric voltage on coupled mechanical-electromechanical responses that govern these complex hexagonal honeycomb-core FG piezoelectric sandwich microstructures.</p>