The size effect of WO3 particles on the X-ray transmission in nano-sized and micro-sized WO3-epoxy composites has been investigated at various synchrotron radiation energies (i.e., 10–40 keV). The results presented in this work demonstrated that the size effect on X-ray attenuation was profoundly dependent on the energy of the synchrotron radiations. The particle size effect was more pronounced at lower synchrotron radiation energies (10–20 keV) since the X-ray transmission in nano-sized WO3-epoxy composites was less than in their micro-sized counterparts. However, this size effect became insignificant at higher energies of 20–40 keV because the X-ray transmissions in both nano-sized and micro-sized WO3-epoxy composites were very similar. The X-ray transmission results for the mammography unit sat between the results of 15–25 keV for XAS beam energies. Meanwhile, the X-ray transmissions in samples for X-ray tube voltages of 40–60 kV of the radiography unit sat between 25 and 40 keV. In addition, for composites with the same filler loading, but with increasing sample thickness, the size effect in X-ray transmission was most prominent for X-ray tube voltages of 25–35 kV but was negligible at 35–100 kV. As the filler loading of the WO3 increased, the mechanical properties showed an initial optimum improvement, but a further increase in the filler loading caused these properties to deteriorate.