The use of high atomic number micro- and nano-particles has been widely extended in biomedical applications. As known, particle size strongly determines most of physical and functional properties when these are infused within biological systems. Monte Carlo simulations are one of the most commonly used tools to investigate physical processes involved in these particle infused systems aimed at radiosensitivity enhancement or theranostic purposes. In this context, it is commonly found that Monte Carlo approaches disregard specific particle dimensions and an "equivalent compound" is used instead. However, establishing whenever physical properties, like fluorescence emission or dose enhancement, might be dependent upon particle size distribution still remains as a significant challenge for Monte Carlo modelling. To this aim, Monte Carlo simulations based on PENELOPE main code were performed for different gold particle sizes' configurations. A volume of gold was kept constant while distributed in different quantities of micro-spherical regions inside a 1 mu l water sphere surrounded by a 1 mm radius water shell. Fluorescence emission was induced by X ray irradiation and the effect of different particles' sizes was investigated recording emergent spectra for K and L fluorescent lines. Besides, an "equivalent compound" was defined in terms of the additivity rule to check if it may or not reasonably represent any complex case of specific particle size distribution. The obtained results indicate an asymptotic trend towards the equivalent compound for the emerging fluorescent lines when particles' size is decreased, thus suggesting that the equivalent compound adequately represents the asymptotic case of "perfect dilution".