CdS semiconductor films of different crystalline size have been grown by a precipitation technique. The crystalline sizes were controlled by the reaction time period, pH/temperature of the solution and thickness of the deposit. From the optical absorption, the band gap of bulk-CdS is found to be 2.405 eV and is increased to 2.97 eV for nanocrystalline samples of average crystalline size 5.0 nm estimated from the blue shift. AFM analysis were performed to estimate the average crystalline size. Photoluminescence studies of CdS nanocrystalline samples show a red shift. The intensity of red luminescence(similar to 1.8 eV) decreases and its peak position shifts to the lower energy upon increasing the particle size. Photovoltage as a function of crystalline size/band gap has been studied using a photoelectrochemical solar cell configuration Ti/CdS/S2-, S-2(2-)/Pt.