In a Proof of Work (PoW) blockchain, each block has a predefined size limit that dictates how many transactions can be included in that block. This limitation is crucial as it directly affects the network's throughput and the speed at which transactions are confirmed. A smaller block size can lead to longer transaction times during periods of high demand, while a larger block size can increase the risk of centralization and strain on network resources. Therefore, careful consideration must be given to the block size to balance efficiency, security, and decentralization.