This study investigates the influence of varying bamboo fibre (BF) sizes on the thermal, mechanical, and physical characteristics of composites made from bamboo fibre and polyurethane foam (PF). BF/PF composites were fabricated using fibre sizes less than 150µm, and within 150–300 µm, 300-425 µm, and 425-600 µm ranges. Thermogravimetric analysis (TGA) identified three primary decomposition phases, corresponding to moisture removal, hemicellulose breakdown, and cellulose degradation. The results demonstrated that incorporating different BF sizes enhanced the composites’ thermal stability. The thermal stability of BF/PF composites was improved; the results show that BFP3 (299.07°C) has a higher onset of degradation compared to neat and other composites. In terms of mechanical properties, larger fibre sizes improved both flexural and compressive strength, with the 300-425 µm (BFP3) size yielding the highest flexural (0.192 MPa) and compressive (0.232 MPa) strengths. However, water uptake, moisture content, and thickness swelling increased with fibre size. Notably, composites using fibres smaller than 150 µm (BFP1) recorded the lowest values in water absorption (42.54%), thickness swelling (3.66%), and moisture content (3.48%).