This is a neat work that tries to answer that very question, and actually got the answer.
Zeuch and colleagues obtained infrared spectra for cluster sizes ranging from 85 to 475 molecules. As expected, there was a shift in the spectrum maxima towards lower wavenumbers as cluster size increased. The transition from 3400 to 3200 cm–1 began at around 275 molecules, with the first crystalline ice occurring in the centre of the cluster, forming a ring of six hydrogen-bonded water molecules in a tetrahedral configuration.You may read the rest of the article on the important implication of this work, especially in understanding ice nucleation.
As the cluster size increased further, the crystalline core gradually grew. By 475 molecules, the infrared spectrum was dominated by the ice structure: the formation of the ice crystal was all but complete. This behaviour matched theoretical predictions made by a different group of researchers in 2004.