The species-range size distribution is determined by speciation, transformation of range sizes, and extinction events. Empirical studies suggested that it has a left-skewed distribution on a log-axis. In this presentation, we propose a general mathematical framework to investigate the dynamics of such a species-range size distribution, by taking these effects into account. Also, we discuss conditions that the model produces the above-mentioned left-skewed distributions. The model developed is flexible and easy to extend, and thus it allows us to examine various existing hypothesis.