Biochar has received significant attention for its potential in bioremediation of polluted soils. However, the impact of biochar particle size is yet to be investigated. We evaluated the role of various particle sizes (<3, 3–6 and 6–9 mm) of commercially available hardwood Acacia arabica biochar applied at the rate of 10 g kg⁻¹ soil in the immobilisation of heavy metals 12 months after application. Heavy metals cadmium (Cd), nickel (Ni) and lead (Pb) were spiked in designated pots from their respective sources CdSO₄, Ni(NO₃)₂ and PbNO₃ according to their permissible limits (0.2–2, 8.1 and 10–15 mg kg⁻¹, respectively). Biochar particles of sizes <3, 3–6 and 6–9 mm significantly mitigated contamination of Cd by 35%, 10% and 9%; Pb by 61%, 60% and 35%; and Ni by 64%, 45% and 3.2%. Relative to the control, application of biochar particles of sizes <3, 3–6 and 6–9 mm significantly increased soil porosity by 10.3%, 4.2% and 3%; saturation percentage by 100%, 42% and 27%; pH by 0.53%, 2.6% and 4%; and organic matter by 33.6%, 19.7% and 16.8%. Soil bulk density decreased by 12%, 5% and 2.3%; and electrical conductivity by 19%, 20% and 24%. The contamination factor for Cd was >1 (in the moderate contamination category), whereas Pb and Ni had contamination factors <1 (in the low contamination category), under biochar application. The heavy metal spiking effect was also significant and ranked as Pb > Cd > Ni for soil pH, electrical conductivity and porosity; Cd > Ni > Pb for organic matter; and Ni > Cd > Pb for bulk density and saturation percentage. Smallest sized biochar (<3 mm) maximally stabilised heavy metals in the soil and improved soil physicochemical properties. Therefore, heavy metal polluted soils should be treated with fine (<3 mm) biochar for maximum immobilisation for heavy metals and improvement in soil physicochemical properties.