Luminescent Solar Concentrators (LSCs) are polymeric devices that can complement the present silicon photovoltaics in buildings as smart windows. For this purpose, luminophores embedded in a polymer converts high-energy UV photons to NIR/red photons and couple it to small photovoltaic devices attached to the edge(s) of an LSC. This work compares the maximum power output of a lanthanide doped red phosphor embedded PMMA based LSC measured at two dimensions—square (5 × 5 × 0.5 cm2) and rectangular (5 × 2.5 × 0.5 cm2). Later, a UV filter was added to the rear of the rectangular LSC. By comparing the overall results, it is evident that out of all variations, the rectangular LSC without the rear filter performs better. This infers that light trapping within the LSC is entirely from the UV photons, which is why the presence of a UV filter diminishes the LSC performance. Besides, by changing the dimension of the LSC sheet, the performance of the silica reinforced 0.75 wt% composite which used to be lower than the 0.25 wt% composite is slightly enhanced while that of the 0.50 wt% is lowered. This means that the reduced path length of the captured photons transforms the LSC’s loss factors. This implies that depending on the geometry and aspect ratio of the LSC, the optimal particle concentration varies. This insight is significant in designing Building Integrated PhotoVoltaics toward zero-energy buildings.