The performance of free-space optical (FSO) communication system is investigated from the information theory perspective, under the influence of atmospheric loss and channel fading due to the turbulence and pointing error. The outage probability and average channel capacity of the FSO link are evaluated using a combined slow-fading channel model and a Gaussian beam wave model, unlike the limiting Rytov-based scintillation model considered in earlier work. Taking into account various link design criteria, our study reveals that lower outage probability can be achieved with larger receiver aperture due to the aperture averaging effect. Optimization of the capacity metrics is best performed through proper selection of the aperture size and beam width for known laser wavelength, particularly for long-distance links and strong turbulence scenarios. Hence, findings and results from our analysis can be treated as the design benchmark for optimal planning and deployment of optical links.