A general correlation for the Xe-129 NMR chemical shift-pore size relationship (delta versus D) in porous silica-based materials over the range 0.5-40 nm has been demonstrated: delta = delta(s)/(1 + D/b), with delta(s) = 116 +/- 3 ppm and b = 117 +/- 8 Angstrom for the 34 materials studied. The correlation maybe used in the characterization of silica samples with unknown pore structure. Even within this general correlation, subsets of materials of similar origin display finer correlations that indicate an acute sensitivity to details of the pore surfaces.