Natural convective heat transfer from flat horizontal isothermal plates has been investigated for more than a century. In the present study, the influence of the size of the plate on the heat flux is investigated. Two ranges of the heat transfer intensity are proposed for this problem: for the plates with characteristic length smaller than 0.1-0.2 m, the heat transfer coefficient is inversely proportional to the plate width;for plates Wider than 0.2 m, the heat flux is not influenced by the plate width. The explanation for the discrepancy between the two ranges of natural convective heat transfer based on analysis and comparison of experimental data available in the literature has been proposed.