2wt％ TiB2/Cu composite powders were fabricated in situ by reactive gas atomization. The fabricated composite powder exhibits high sphericity, and the powder sizes range from 5 μm to 150 μm. The morphology of the Cu matrix and the distribution of the TiB2 particles in the composite powders vary with the powder size. The critical transitions of interface morphologies from dendritic-to-cellular and cellular-to-planar interfaces occurs when the composite powder sizes decrease to 34 μm and 14 μm, respectively. Compared with pure Cu droplets, the composite droplets undergo critical transition of the interface morphologies at a smaller droplet size corresponding to a higher cooling rate because the existence of TiB2 particles can cause instability in the advancing solidification front and heterogeneous nucleation. With decreasing powder size, the extent of the TiB2 particle interdendritic segregation decreases as the result of enhanced engulfment of TiB2 particles by the advancing solidification front.