This study investigated the effect of expanded porous glass bead particle size on the mechanical, surface, and optical properties of low-density polyethylene (LDPE) composites. Two granulations of the filler with a range of particle sizes were inserted into the LDPE matrix at 10% wt. The composites were characterized using static tensile tests, under sudden impact loading, surface roughness measurements, and colourimetric analysis on the CIELAB scale. The results showed that filler significantly modified the mechanical performance, surface morphology, and optical properties of LDPE. The filler reduced tensile properties and increased surface irregularity compared with the unfilled polymer. Clear changes in colour parameters were also observed, including lowered lightness and changes in chromatic coordinates. The magnitude of these changes depended on filler particle size. The results show that filler particle size is an important factor influencing the mechanical, surface, and optical properties of LDPE composites and should be considered when designing materials that require a balance between mechanical performance and surface appearance.