Quantum size effect (QSE) comprises a novel phenomenon where structural, mechanical, thermal, electronic and optical properties of solids are affected by the reduction in particle size. Size-dependent properties in semiconductors and dielectrics come into play especially by making one (thin films) or more (quantum wires and dots) dimensions of a sample very small, particularly going from micro to nano-dimensions. One or more dimensional QSE is accompanied with an increase of the light absorption and the blue-shift of the optical band-gap due to a creation or reduction of the crystallite sizes having different dielectric constants (relative permitivity). Understanding the nature of the size-induced properties is of fundamental importance for advanced technological applications. The size-dependent band-gap makes a material attractive for optical absorption-based applications. In this work, a-Si:H films different in thickness were prepared by PECVD technology using different hydrogen-diluted silane. One-dimensional QSE (when film thickness decreased) and three-dimensional QSE (when due to a high hydrogen-silane dilution the nano-crystalline structure of the films appeared) were observed.