Polystyrene structures, consisting of a single chain of 40, 62, 171, or 364 monomers folded into periodic cells, were generated by a new Gaussian lattice method. The different structures were simulated to study system size effects on the calculated excess chemical potentials of five gases (Ar, O-2, Na, CH4, and CO2) in amorphous polystyrene glass. Excess chemical potentials were calculated using the test-particle insertion method and a modified excluded-volume map sampling algorithm. For the largest structure, we observe that the excess chemical potential decreases linearly with the depth of the Lennard-Jones well of the penetrant, in agreement with experiment. oHwever, we find that the smaller structures are, on average, unable to form cavities of sufficient size to accommodate CH4 and CO2 (the larger penetrants). We also show that the instantaneous excess chemical potential distribution provides a sensitive probe of system size effects on polymer architecture.