It is shown, that the Doppler line from a single large particle moving with a constant velocity through a finite- width laser beam, undergoes a doublet-type splitting under specific observation conditions. A general requirement is that particle size 2a is not negligibly small, compared with beam diameter 2w$0. Three optical mechanisms of line splitting are considered. The first one is based on nonsymmetric diffraction of a bounded laser beam by a moving particle. The second arises from the transient geometry of diffraction. The third mechanism, of photometric nature, originates from specific time variation of total illuminance of moving particles when 2a>Lambda, the interference fringe spacing in the measuring volume. The diffraction splitting is observed when a detector is placed near one of diffraction minima corresponding to either of probing beams, and 2a equals (n0.5)Lambda for n equals 1,2. The photometric splitting is observed with an image-forming optics, when 2a equals n(Lambda) . That gives the possibility of distant particles sizing based on the Doppler line splitting phenomenon. A general theory of line splitting is developed, and used to explain the experimental observations quantitatively. The influence of the scattering angels and observation angle on the line splitting characteristics is studied analytically and numerically.