The influence of size limitation of the strongly scattering medium and its boundary reflection on the spatial distribution of pump and random lasing radiation density was calculated with Monte-Carlo method. This limitation results in formation within the sample area with increased concentration of the lasing photons what is conditioned by selection of higher-Q compacted trajectories. The area location and the photon density in it are strongly depended on the sample thickness and scattering efficiency. The boundaries reflection partially smooths of photon density distribution and rises its value. It gives optimal medium size providing maximal lasing energy, which proves experimentally.