<p>Copper nanoparticles are widely studied for catalytic applications; however, controlling their aggregation during synthesis remains challenging due to the complex interplay of synthesis parameters. In this work, we systematically investigate the aggregation behavior of copper nanoparticles synthesized via a one-pot chemical reduction method at room temperature by independently varying key synthesis parameters, including copper precursor concentration, reducing agent concentration and injection rate, capping agent concentration, and overall component concentrations. Electron microscopy-based size analysis combined with statistical evaluation reveals that, under the studied conditions, the primary particle size remains largely insensitive to changes in copper precursor concentration, while aggregation is strongly promoted with increasing precursor concentration and total component concentrations. In contrast, the reducing agent plays a decisive role in both particle growth and aggregation. Increasing its concentration promotes the formation of larger primary particles and aggregates, whereas faster injection rates effectively suppress aggregation over a broad parameter window. The capping agent, poly(vinylpyrrolidone), exhibits a comparatively limited influence on aggregation and no pronounced effect on primary particle size under the conditions investigated. The direct impact of aggregation on catalytic performance is demonstrated using CO oxidation as a probe reaction, where a decrease in activity is observed with increasing aggregate size despite similar primary particle sizes. Overall, this study establishes aggregation as a highly sensitive and independently tunable outcome of copper nanoparticle synthesis, providing practical guidelines for controlling aggregate formation during scale-up and for the rational design of nanostructured copper catalysts where aggregate architecture is critical to performance.</p>