The modal distribution at the output of a multimode optical fiber behaves as a speckle pattern. Further, a speckle pattern carries information on the pupil diameter of the optical system exit aperture. Pupil measurements by speckle size determination may not be an easy task, because of the statistical nature of the speckle phenomena. The measurement of a multimode fiber core is proposed using a speckle autocorrelation algorithm to obtain the minimum speckle diameter present in the pattern generated by the fiber. Both computer simulations and actual pupils are used to test its performance. Core diameter measurements are carried out showing good agreement with the specifications supplied by the fiber manufacturer.