The size parameters of an air vessel, mainly including its initial gas volume and total volume, account for its performance in hydraulic transients. Traditionally, the size of an air vessel in a given water supply system is estimated by trial and error, usually aided by computers. To simplify the preliminary estimation of the air vessel's volume, in this paper, an approximate analytical method is proposed and derived on the basis of incompressible flow theory and simple harmonic vibration theory. With this method, a theoretically optimal size of an air vessel, which is able to meet the protective requirements in hydraulic transients, could easily be obtained in an analytical way. Besides, a real engineering case is introduced in this paper to illustrate that the proposed method could give guidance and reference in the preliminary work of designing an air vessel in practice.