Light collection efficiency from BGO crystal scintillators of various sizes and shapes was measured by reading them using 1×1cm2 avalanche photo diodes. When the crystals have simple geometry, the light collection efficiency was found to depend on their size, shape and the read-out position through a rather well-defined empirical scaling relation. The light collection efficiency of tapered crystals was seen to depend on both the position of γ-ray irradiation, and the read-out position of the avalanche photo diodes. Using optical Monte-Carlo simulations, the relation was reproduced assuming plausible proper parameters for surface conditions and the attenuation length. This results were reproduced with a reasonable accuracy by optical Monte-Carlo simulations. Simple physical explanations are given to these geometrical effects. •We studied light output of different shapes and size BGO crystals with an APD.•With a Monte-Carlo simulation, we successfully reproduced measured light outputs of BGO.•We derived a scaling law of a rectangle crystal volume and surface area APD attached.•We measured a dependence of the light output on a position with non-rectangle crystal.