The characteristic size of a collimated Gaussian beam propagating through 1-13 km atmospheric paths is investigated by simulating phase screens using the fast Fourier transform method. Taking a threshold into account, a method to derive a modified centroid and corresponding characteristic radii of the short-term spots is proposed. Effective radius, robust radius, sharpness radius, and maximum radius are analyzed by probability statistics. Furthermore, several parameters representing the energy content of the spots within each radius and the energy duty cycle of the maximum radius are studied. The study shows that, when the modified centroid is taken as a center, the effective radius is more suitable for application after a long propagation path, while the maximum radius is more effective for a short distance. However, when all effective subspots of a short-term image are investigated, the maximum radius is usually utilized, and the energy duty cycle represents the effect probability.

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