In this study, the size effect of Cu₂O nanospheres on femtosecond laser reductive sintering/melting properties for Cu printing is investigated. First, two types of monodispersed Cu₂O nanospheres (average sizes of 110 and 250 nm) are prepared using the polyol method. The particle size is significantly affected by the amount of polyvinylpyrrolidone because of the capping of initially-precipitated nanospheres and their aggregations. Both the prepared inks containing nanospheres and reducing agents exhibit the same reaction temperature in thermogravimetry and differential thermal analyses. These results indicate that the size of the nanospheres does not affect the reaction temperature. The small Cu₂O nanospheres are well-sintered, whereas the large nanospheres are easily melted by irradiation with femtosecond laser pulses. As both inks possess the same reaction temperature, their printing properties depend on their thermal conductivity. The low thermal conductivity of the large nanosphere ink owing to their small reduced surfaces against volume induces local heating and subsequent melting, and the high thermal conductivity of the small nanosphere ink owing to their large reduced surfaces against volume increases sintering area. The sintered patterns fabricated using the small nanospheres exhibited an electrical conductivity of ∼1.1 × 10⁶ S/m, which is 1/50 of pure Cu. These strong size effects of the nanospheres on direct writing demonstrate that mixed sizes of the nanospheres can improve the electrical conductivity of the patterns because of their high density and sufficient thickness.