Vortex-induced vibrations (VIV) is a well-known fluid flow phenomenon studied in multiple engineering disciplines and typically sought to be eliminated or minimized. This study aims to investigate the effect of bluff body cylinder sizes on VIV based electrical energy converter from low velocity flowing water. This study is particularly important because the VIV based technology explores the vast untapped new sources of clean and renewable energy from rivers and oceans with current velocities as low as 0.514 m/s to 1.8 m/s which were previously off limit. Computer model was created using solid modelling computer aided design an engineering software and exported to ANSYS for fluid-structure interaction simulation. Three cylinder sizes, 50 mm,75 mm, and 100 mm were tested at five different velocities of between 0.514 m/s, 0.7 m/s, 1.0 m/s, 1.4 m/s, and 1.8 m/s, respectively using computational fluid dynamics (CFD) simulation. The simulation result shows that the maximum heaving force amplitude of 28N emanates from the 100mm diameter at a current velocity of 1.8m/s.