Total charge acquired by soda-lime glass particles of two different sizes in nitrogen environment were measured in vibrated beds. A systematic difference of surface charge density was observed. Complementary Discrete Element Method simulations were performed to examine how this size dependence could be captured in a tribocharging model. Neither dielectric breakdown of the gas nor charge relaxation could explain the size dependence. A mechanistic model is not developed in this study, but experimental charge levels could be captured by allowing the effective work function (EWF) to be different for two particle sizes. The same EWF difference between the two sizes could capture the results obtained in vibrated beds of two different dimensions and two different wall materials. This approach was applied to examine the charging behavior of a bidisperse system studied experimentally. Their results could be reproduced if size dependent EWF is coupled with stochastic variation within each particle size.