We use the nonequilibrium molecular dynamics method to investigate the thermal conductivity of graphene by changing size in each dimension simultaneously (here we use a square graphene model). We also consider the extreme case of one dimension using the infinite wide and quite narrow graphene models. Our results show the thermal conductivity of these three graphene structures increases with the system size and the rate of increase obviously goes down as increasing the systems size. Then by comparison, we find that the thermal conductivity of the infinite wide graphene is larger than those of the other two graphene models, and the narrow case has the lowest thermal conductivity. This is because there exist edge-localized phonons (edge effect) and boundary phonon scattering in the square and narrow graphene model, and the latter has stronger edge effect and boundary phonon scattering due to the narrower width. Thus the calculations for thermal conductivity of infinite wide graphene may give the upper limit values of the thermal conductivity in graphene. So we further calculate the infinite wide graphene's thermal conductivity at larger scale, it is found thermal conductivity keeps increasing in a wide range of length and finally saturates to a finite constant at greater than 10. Based on the phonon spectral energy density (SED) method, we attribute this convergence behavior to the graphene ZA modes. These findings provide knowledge for tailoring and engineering of the thermal conductivity of graphene in thermo-electronic and photonic devices.