We present a first-principles study of the structural, electronic, and optical properties of hydrogenated amorphous silicon (a-Si:H). To this end, atomic configurations of a-Si:H with 72 and 576 atoms respectively are generated using ab initio molecular dynamics, where the larger structures are defect free, closely matching the experimental situation and enabling the comparison of the electronic and optical properties with experimental results. Density functional theory calculations are applied to both configurations in order to obtain the electronic wave functions. These are analyzed and characterized with respect to their localization and their contribution to the density of states, and are used for calculating ab initio absorption spectra of a-Si:H. The results show that both the size and the defect structure of the configurations modify the electronic and optical properties and in particular the value of the band gap. This value could be improved by calculating quasi-particle (QP) corrections to the single-particle spectra using the G0W0 method. We find that the QP corrections can be described by a set of scissors shift parameters, which can also be used in calculations of larger structures. The analysis of individual contributions to the absorption by evaluating the optical matrix elements indicates that strong localization enhances the optical coupling, but has little effect on the average transition probability 〈|vcv|²〉 for which we find a dependence E²+const on the photon energy E, irrespective of the nature of the initial or final state.