A theoretical void-size distribution in random packings of equal-sized spheres, defined as the distribution of the probability of finding a spherical void of a given size, is derived. In order to assess its validity, the three-dimensional distribution is used to evaluate, by an approximate procedure, the size probability distribution of the two-dimensional voids appearing in an arbitrary section of the bed. The predictions of the model, which contains no fitting parameter, are in close agreement with the experimental results obtained from real packed beds.