We measure the sintering behavior of thin and thick films of ceramics made by stacking several layers of tapes. The stack thickness was varied from 0.12 to 0.50mm. A powder-pressed dog-bone sample, 1mm thick served as the baseline. While all specimens exhibit flash sintering, the following differences are noted. (a) The onset of flash, which has been shown to occur within a narrow band of volumetric power density for powder-pressed dog-bone shaped specimens, moves to a higher value for the thinner specimens. (b) The power transition collapses, again, to within a narrow band when the power density is normalized with respect to the surface area rather than the volume. (c) Sintering of thinner specimens to full density required a higher current-density limit. These results point to a deeper significance of black body radiation than its use in estimating the specimen temperature in a steady state of flash that is calculated from the energy balance between electrical and radiated energy. The experiments were carried out on 3mol% yttria stabilized zirconia at constant heating rate