Zero-valent iron (ZVI) nanoparticles (<100 nm) have garnered significant interest for their potential in remediating metal-polluted soils. However, it remains unclear whether they offer demonstrable advantage over ZVI microparticles (e.g., iron powder or grit) for in situ metal immobilization. This field study compared the effectiveness of ZVI micro- and nanoparticles for immobilizing metals and promoting perennial ryegrass (Lolium perenne L.) growth in an acidic Podzol (pH 4.5) contaminated by a nickel–copper smelter. ZVI nanoparticles were applied as an iron–biochar composite synthesized via pyrolysis. Amendments were incorporated into the soil for 16 months under field conditions. Subsequently, soil samples were collected and used in a 21-day laboratory experiment, comparing ryegrass growth in amended and unamended soils. While ZVI nanoparticles were more effective than microparticles at immobilizing copper, neither treatment significantly reduced nickel concentrations in the soil solution. Consequently, neither ZVI amendment substantially improved ryegrass growth, likely due to persistent nickel phytotoxicity. The study provides a direct field comparison of ZVI micro- and nanoparticle efficacy for metal immobilization in contaminated soils.