The theoretical analysis of a probing laser beam interaction with a fluid drop is carried out. It is shown that moving droplet Doppler signal amplitude depends on position in Gaussian beam. Possibility of bubbles/droplets sizing by determination of Doppler signal extremums positions is shown. The glass balls radiuses (1.75 mm and 5 mm) are measured. The radiuses sizing relative errors are equal 1.1% and 8% accordingly. The radiuses of the bubbles floating in viscous fluid are measured. Thus, it is shown that the offered sizing method is perspective in two-phase experimental fluid mechanics.