Al:Zr composite powders are synthesized via arrested reactive ball milling (ARM) and contain small inclusions of Zr within an Al matrix. We characterize the microstructure and ignition temperatures of the powders as a function of size using the following size ranges: 0-10, 10-32, 32-53, 53-75, and > 75 mu m. We observe uniformity in density for all size ranges but the largest (> 75 mu m), implying consistent elemental composition, and we observe elongation of Zr inclusions within particles of the larger three size ranges. Williamson-Hall analyses of Zr XRD patterns indicate similar Zr grain sizes across all powder size ranges and negligible residual strain and interfacial energies within the composite powders. TGA shows an increase in oxidation as particle size decreases, which we attribute to surface oxidation through comparison to an analogous Zr-rich composite powder. The onset of ZrO(2)formation is compared to that of Al-Zr intermixing and intermetallic formation, and we find that the latter corresponds well to the measured ignition temperature. Unlike pure metal fuels, where ignition is governed by the onset of oxidation and ignition temperatures typically increase with particle size, these particles ignite at much lower temperatures (< 350 degrees C) with no dependence on size for particles less than 75 mu m. Atomic intermixing and the formation of intermetallic and Zr compounds are the primary drivers for ignition in this system at slow to moderate heating rates, and we attribute the similar ignition temperatures across multiple particle sizes to the shortest diffusional intermixing distances being comparable in each size range.