Power system interconnections bring primary economic and technical advantages. However, certain technical problems arise from the increasing size of the synchronous grid, such as centralized power dispatching and inter-area oscillations. From the technical viewpoint, this paper proposes three basic constraints to determine a reasonable size of synchronous grids, which are the synchronizing support effect disappearance constraint, the minimum oscillation frequency constraint of low frequency oscillations and the frequency stability constraint. First, the research results indicate that synchronous power systems would lose the essential power support advantage when synchronizing support effect disappears, which requires the synchronous system operated in direct synchronism, i.e. relative rotor angles all below 90 degree within the grid. Secondly, the total damping of generators with PSS tends to be negative when the frequency is lower than 0.3 Hz, and the lower frequency the worse damping effect. Thirdly, the frequency stability constraint depends on steady state frequency deviation requirement and transient state frequency deviation requirement. Based on general structures of power systems, the triangle criterion composed of the three constraints could provide useful references for regional or national synchronous power system interconnections and power system planning.