A hadron spectrum calculation with two light dynamical quark flavors was carried out with the Kogut-Susskind quark action at β=5.7 on lattices of spatial size 83, 123, and 203 for mq=0.01 and 0.02 in lattice units, with emphasis given to a systematic study of the finite-lattice-size effect. It is found that hadron masses on a 163 spatial lattice at this β still suffer from a significant finite-lattice effect at least for mq=0.01, showing the importance of a quantitative control over the finite-size effect in comparing simulation results with the experimental hadron masses even for a fairly large lattice. A comparison is also made to the analytic prediction for the finite-size effect from chiral perturbation theory.