We investigate guided acoustic-wave Brillouin scattering (GAWBS) in single-mode optical fibers. By treating the discrete GAWBS spectrum as a continuum, we propose a closed-form formula to evaluate the GAWBS variance due to radial modes, showing its dependence on the inverse of the fiber effective area. The formula is checked against an experimental validation at variable signal power. The almost flat experimental results with power suggest that GAWBS dominates stimulated electrostriction. We also compare GAWBS with the Kerr effect showing their common grounds and their main differences. The phase noise characteristics of the nonlinear interference induced by GAWBS is investigated, showing that in ultra-long links GAWBS can be safely taken as a circular white noise. The theoretical results can be used for quick estimations of the GAWBS penalty with any fiber effective area. In particular, we estimated a GAWBS penalty on the signal-to-noise ratio (SNR) of 0.5 dB after 6000 km with fibers of effective area 150 μm2.