Using spark plasma sintering (SPS), Mn₃Cu_0.6Ge_0.4N crystallites have been fabricated with different crystallite sizes, and their magnetic properties and thermal behaviors were systemically investigated. With decreasing crystallite size, the magnetic transition becomes increasingly slow, accompanied by broadening of the negative thermal expansion (NTE) operation-temperature window. The NTE operation-temperature window for the 12-nm crystallite sample reaches at 140 K, which is about 75% larger than that of the 74-nm crystallite sample. The magnetic properties and NTE operation-temperature window can be tuned by varying the crystallite size. This discovery will promote an even wider range of practical applications in precision devices.