We present a rigorous description of finite-size effects for a large class of models with an asymmetric first-order transition, assuming that all phases contributing to the transition have a finite correlation length. If the model describes the coexistence of two phases, it is shown that, at sufficiently low temperatures, the shift of the transition point due to finite-size effects in a volume L(d) with periodic boundary conditions is O(L-2d), in contrast to certain claims in the literature. We also discuss different ways to determine the transition point from finite-size data, which involve only exponentially small systematic errors in L.