To improve the capability of the single-electron transistor (SET) for the practical application, it is inevitable to make the researches under the condition of room temperature. Initialized from the orthodox theory, this research calculated the critical size of coulomb island of SET which can work normally at room temperature: the memory device is 6.5nm, the logic device is 1.5nm; it also calculated the critical size of coulomb island of SET when the energy quantization effect at room temperature, 4.7nm, appeared. Furthermore, it verified and analyzed these two kinds of critical size. In addition, through a comparative analysis, it drew conclusion that all logical devices must consider the energy quantization effect and all memory devices should consider the energy quantization effect at room temperature. The analysis result shows that determination for critical size of coulomb island has important significance for the application of SET.