Nuclear magnetic resonance measurements of Xe-129 chemical shifts in a large variety of nonionic calibrated microporous and mesoporous silica glasses with open porosity provide a calibration of spectroscopic mesopore size measurements. The pore size dependence of the xenon chemical shift is quantitatively interpreted,by considering both fast exchange and van der Waals interactions at the surface, which cause chemical shifts. The temperature dependence of the chemical shift supports the proposed model and characterizes the enthalpy of adsorption at the pore surface. The xenon NMR measurement is also useful for very low or even quasi-closed porous systems and may provide assessment of the macroscopic xenon diffusion coefficient in xerogels.