The kinetics of isothermal crystallization at the free surface of dense, 150-1050 bilayer (BL) (similar to55-385 nm) thick amorphous solid water (ASW) films is investigated experimentally, and a model accounting for the observed thickness dependence is proposed. We find that as the ASW film thickness is increased above 150 BL, surface crystallization accelerates, rapidly at first and then more slowly until essentially size-independent kinetics are attained by 1050 BL. The potential origin of this thickness dependence is elucidated by a geometrical model of surface crystallization that we formulated using mechanistic information deduced from available experimental data. This simple mean-field model predicts that as film thickness is reduced below some critical value, the number of grains contributing to surface transformation progressively decreases, forcing each grain to convert a larger surface area and thus slowing crystallization. Good agreement between experimental data and the theory is realized using only two thickness-independent kinetic parameters (per temperature), suggesting that the model describes the basic physics of crystallization in these ASW films. Nucleation and growth rates determined via model fits range from similar to5.5x10(9) cm(-3) s(-1) and similar to0.3 Angstrom/s at 136 K to similar to6.5x10(10) cm(-3) s(-1) and similar to1.5 Angstrom/s at 140 K, corresponding to activation energies of 100 and 68 kJ/mol, respectively. (C) 2003 American Institute of Physics.