The valence photoelectron spectra of water clusters are studied experimentally and by ab initio calculations. The size dependence of the vertical ionization energy of the outermost orbitals is explicitly shown. A shift toward lower values is observed. For small cluster sizes, it can be rationalized as an effect of charge delocalization as the system is becoming more extended. Ionization energies of larger clusters decrease linearly with inverse cluster radius and asymptotically approach the value of liquid water. In the calculations, we apply a reflection principle approach based on sampling a quantum mechanical distribution of different initial-state geometries to clusters. An excellent agreement of peak shapes calculated thus with measured ones is shown. Using additional polarization fields, the extension of this approach to the photoionization of liquid water is demonstrated. Upon deuteration of the water clusters, we experimentally and theoretically find slightly larger absolute values of the vertical ionization energies. We suggest that the measurement of electron ionization energies can be used as an alternative means to characterize water cluster sizes, which can complement the use of scaling laws.