Experimental studies of grain boundary invasion by a wetting fluid give clear evidence for a size dependence. In order to get a better numerical insight into grain boundary (GB) wetting as a percolation process, we have investigated size effects during gallium penetration into quasi-2D zinc polycrystalline strips of various width and during water penetration into 3D cylindrical NaCl polycrystals. Both systems are likely to be good objects for studying percolation effects because of a random distribution of wettable GB's. Computer simulation on the square lattice, with a "wetting" probability p = 0.60 close to the number of experimental points (several dozens), shows a striking resemblance between both sets of data. Making more runs (about 10(5)) demonstrates consistency of our model with an earlier reported work by Marrink and Knackstedt describing finite size effects in elongated lattices. Using their approach, an excellent agreement can be obtained between the experimental and simulated data, as well as between the latter and theoretical predictions. (C) 2005 Springer Science + Business Media, Inc.