Batch size (or lot size) is the quantity of a product made in one run before switching to another. Large batches spread changeover cost over more units but inflate WIP, lengthen lead time, and hide defects.
Lean strives toward ever-smaller batches — ideally one-piece flow — which is only economical once changeover time is driven down through SMED. The economic trade-off for purchased parts is captured by the EOQ model.