A computational model, which can reveal the out-plane size variation of sandwich panel unit cell, is proposed to study the out-plane size effects of honeycomb sandwich panels. In this model, the three dimensional unit cell of sandwich panel, consisting of the upper and the lower skins and the homogenized core, is constructed based on homogenization method. Three methods, i.e., homogenization method, the finite element method and the classical laminated plate theory, are used to study the influences of the out-plane size variation on the bending effect and vibration response of sandwich panel. Numerical results show that the solution of finite element method agrees with that of laminated plate theory when the number of out-plane unit cell is small. However, once the number of out-of-plane unit cell is large enough, the finite element solution is close to the homogenization results.