Silica gel is widely used in commercial applications as a water adsorbent due to its properties including hydrothermally stable, high water sorption capacity, low regeneration temperature, low cost and wide range of pore diameters. Since the water sorption capacity of silica gel strongly depends on the pore size and structure, which can be controlled during synthesis, this paper study the effect of pore shapes and dimensions of silica gel upon the adsorption of a water molecule aiming at maximising the water sorption capacity. In particular, we consider three types of pore structures, namely cylindrical, square prismatic and conical pores. On using the Lennard-Jones potential and a continuum approximation, we find that the minimum radii for a water molecule to be accepted into cylindrical, square prismatic and conical pores are 4.009, 3.7898 and 4.4575 Å, respectively. For cylindrical and square prismatic pores, the critical radii which maximise the adsorption energy are 4.5189 and 4.1903 Å, respectively. Knowledge of these critical pore sizes may be useful for the manufacturing process of silica gel that will maximise the water sorption capacity.