Distributed sensing nodes in the Internet of Things (IoT) require efficient and adaptable electromechanical transduction mechanisms for harvesting low-frequency micro-vibration energy. Macro Fiber Composites (MFCs), exhibiting high piezoelectric coefficients and exceptional flexibility, constitute promising material solutions for this purpose. However, their performance is constrained by non-uniform internal electric field distributions and an incomplete understanding of the relationships between critical structural parameters and output characteristics. To address these challenges, a multi-scale parameter optimization framework integrating finite element simulation with experimental validation is proposed. A Representative Volume Element (RVE) model is developed to elucidate the non-uniform electric field distribution and reveal the pivotal role of the ‘electric-field activated zone’. Building upon these insights, a comprehensive three-dimensional cantilever beam model is constructed to systematically investigate the effects of fiber width and height on stress and electric potential distributions. Finite element analyses indicate that an MFC energy harvester with optimal fiber dimensions of 60 mm (length) × 0.35 mm (width) × 0.35 mm (height) achieves enhanced output voltage while concurrently minimizing stress concentration. Guided by these optimal parameters, a prototype MFC energy harvester is fabricated and experimentally characterized. Under harmonic excitation at the resonant frequency of 8.9 Hz with an acceleration amplitude of 0.5 g, the prototype exhibits an open-circuit voltage of 43 V and a peak power density of 0.31 mW/cm² at the optimal load resistance of 2.5 MΩ. Excellent agreement between numerical predictions and experimental measurements is achieved, with a resonant frequency deviation of only 1.1%. This study establishes optimal dimensional parameters for MFC-based low-frequency micro-vibration energy harvesting and presents a robust design methodology for high-performance, customizable energy harvesting systems.