The buffer size decision for the Flexible Transfer Line (FTL) gains more and more importance because of growing FTL complexity. The buffer size in front of the bay of machines in the FTL is still one of the major optimisation problems faced by production engineers. In this article, we attempt to find the optimal buffer size for FTL with Rework Paths (FTL-RP) that achieves the best throughput of the FTL-RP. A Genetic Algorithm (GA) is presented to find buffer size of FTL-RP. To estimate FTL-RP performance, the well-known decomposition approach is used to find the throughput of the FTL-RP at a given buffer size. In order to achieve the efficient use of the GA, we propose the Multiple Vectors Distribution Method (MVDM) for the genes arrangement.