The crystal size in accretions grown in an icing wind tunnel at ambient temperatures −18≥Ta≥−27°C and different substrate temperatureTs<0°C is studied. For constantTs the mean area\(\bar \sigma \) of crystal sections shows an approximately exponential dependence onTa which may be written as\(\bar \sigma = a\exp [b/\Delta T^2 ]\), where ΔT≃−Ta represents the droplet supercooling. On the other hand, for fixedTa, the crystal size is observed to decrease markedly withTs. By considering previous results on the orientation of crystals forming the accretions, it is shown that the value of the mean angle\(\bar \varphi \) formed by the crystalc-axis with the growth direction depends as well both onTa andTs. The exponential dependence of\(\bar \sigma \) andTa and the observed correspondence between the behaviour of\(\bar \sigma \) and\(\bar \varphi \) are correlated with a nucleation mechanism, accompanied by a process of orientation selection, which would be responsible of both the crystal size and the orientation. The application of the crystal structure analysis to the determination of the growth conditions of hailstones is discussed.