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Gas atomization is the most used powder production technique since it provides good control on particles shape, surface oxidation and dimension. It is a rapid solidification technique involving fast cooling rates, which are strictly correlated to particle size. This relationship is typically described with a power law function that can be determined experimentally by measuring the microstructural length-scale or through the application of a heat transfer model. Both paths were exploited in the present work focusing on a gas-atomized Al-4.5Cu alloy. Atomized powders were characterized by means of X-ray diffraction, differential scanning calorimetry, light and scanning electron microscopy to investigate the relationship between cooling rate and microstructure length-scale. A recently proposed semi-empirical model was validated and discussed in terms of a physically based heat transfer approach. The change in gas-to-melt mass flow ratio (GMR) was also investigated showing that it does not affect appreciably the relationship between solidification rate and particle size, but does increase the Cu supersaturation in the powders of finer size.