Automated assembly lines are modeled as finite open queueing networks and a heuristic for buffer space allocation within these lines is presented. The Expansion Method, an analytical technique for modeling finite open queueing networks and Powell's unconstrained optimization procedure are integrated in a design methodology, which evaluates alternative line topologies, system throughputs, and their optimal buffer sizes. The resulting design methodology is demonstrated for series, merging and splitting topologies of automated assembly lines with balanced and unbalanced service rates.