The frequency of optically excited plasmon oscillations in a metal cluster is studied as a function of cluster size. The model is based on the fact that a sphere filled with free-electron gas forms a cavity with size-dependent eigenfrequencies of the optically excitable TM plasmons. Excitation of plasmon resonances by light in clusters with appropriate radius, and the size-dependent radiative damping of the excited oscillations are studied. Dependence of the frequency of plasmon oscillations and of the dumping on the relaxation rate of electron motion in the metal is discussed. Size-dependent total scattering cross section resulting from the Mie theory is analysed as a function of the cluster radius. The contributions to the scattering of the secondary fields due to excitation of collective electron oscillations and eddy currents are considered. The study is illustrated by the example of sodium clusters interacting with the light field with wavelength of 488 nm. It is shown that the collective motion of free electrons can be excited when clusters approach the radii of 55 nm (l=1), 118 nm (l=2),... respectively, which are the cluster radii available in the experiment on light-induced clusters.