An efficient and reliable pressurization system for an oxidizer and fuel of a liquid propellant rocket is critical for a successful launch. A liquid helium pressurization system employing a heater can reduce its mass, and be made simpler and more reliable than conventional pressurization systems. The key issue to minimize the total mass of the system is the optimization of the size of the liquid helium tank. In this paper, we describe a method to determine the optimal size of a liquid helium tank, and present one set of results under a given set of requirements. In this pressurization system, the heater design is represented by the heater efficiency. To estimate the heater efficiency, the convection heat transfer coefficient should be known beforehand. The guideline how to estimate this convection heat transfer coefficient based on the preliminary experimental data is also presented in this paper.