Since the invention of split sphere multianvil apparatus, the maximum sample size that can be obtained on tungsten carbide (WC) anvils with a certain size has always been a neglected issue. The maximum volume of loaded gaskets and the maximum multianvil cell assembly operating on given size WC anvils are calculated by a simplified geometric model. As a concrete example, the maximum volume of loaded gaskets in different assemblies (18/11, 25/17 (octahedral edge-length/anvil truncation edge-length, in millimetre)) and different initial shapes of octahedrons (octahedron chamfered or not) running on WC anvils with 25.4 mm edge-length are calculated. The maximum initial sizes of gaskets in different assemblies are also calculated according to the maximum volume of loaded gaskets. Using the pyrophyllite octahedron as the pressure transmission medium, the pressure calibration of 14/8, 18/11, and 25/17 assembly are carried out on WC cubes, of which the edge-length is 25.4 mm, to verify the calculated results. The cell pressure is calibrated by in situ observing resistance change caused by phase transition of pressure calibration material at high pressure. All assemblies run stably and no blow-out phenomenon occurs. The experimental results show that the pressure transfer efficiency of 18/11 and 25/17 assembly with chamfered octahedron are better than previous reported results in the same pressure range. The experimental results also indicate that the calculations of the gasket in this work are valid. And the 25/17 assembly with chamfered octahedron can run stably on WC cube with 25.4 mm edge-length. In addition, using graphite tubes as heater, heating structure designs of different assemblies (14/8, 18/11, 25/17) are completed, in which the 14/8 assembly can achieve the highest temperature of 2300 ℃. The diameter of sample chamber is almost one centimeter in 25/17 assembly. This work contributes to the development of assembly techniques for secondary anvil made of polycrystalline diamond materials to obtain larger sample volumes under higher pressure conditions.