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Hot isostatic pressing (HIP) is a key powder-metallurgical route for producing high-performance Cu-Cr-Nb bulks that combine high thermal conductivity with high strength. Because the HIP process largely preserves features inherited from the feedstock, powder characteristics—particularly particle-size distribution—can strongly influence densification behavior, microstructure, and resulting properties of bulks. Here, Cu-Cr-Nb powders from the same batch were sieved into five size fractions (<15 μm, 15–53 μm, 53–105 μm, 105–150 μm, and a full-range 0–150 μm blend) and consolidated under identical HIP conditions to systematically quantify particle-size effects. After HIP, most particles retained a near-spherical morphology, and prior particle boundaries (PPBs) remained pervasive. Grains in interparticle regions were finer than those in particle interiors, and PPBs were decorated with Nb-rich second-phase. Bulks produced from finer powders achieved slightly higher densification with sparse, near-spherical pores, whereas those produced from coarser powders exhibited marginally lower density and occasional unconsolidated defects. Powder particle size primarily affected hardness and strength: bulks produced from <15 μm powder exhibited >10% higher hardness and strength than those produced from 105–150 μm powder. In contrast, ductility and electrical conductivity were comparatively insensitive to powder size, with elongation exceeding 35% and conductivity over 80% IACS. Notably, the strength advantage of fine-powder bulks persisted after aging, cold rolling, and high-temperature heat treatments. Solid-solution and precipitation strengthening were limited because the Cu matrix was essentially solute-free and nanoscale Cr precipitation was not evident, while the remaining submicron second phase contributed only marginally. Instead, dislocation strengthening (∼60%) and grain-boundary strengthening (∼26%) dominated the yield strength and accounted for the majority of the strength differences among the HIPed bulks. Electrical-transport calculations further showed that variations in grain-boundary and dislocation densities reduced conductivity by only ∼1%–2% IACS, explaining its weak dependence on powder particle size. Overall, this work provides practical guidance for feedstock selection, process control, and cost-effective powder utilization in powder-metallurgy manufacturing.