Size-dependent development of the hydrogen bond network structure in large-sized clusters of protonated water, H⁺(H₂O)_n (n=4 to 27), was probed by infrared spectroscopy of OH stretches. Spectral changes with cluster size demonstrate that the chain structures at small sizes (n≤~10) develop into two-dimensional net structures (~10<n<21), and then into nanometer-scaled cages (n≥21).