A Bitcoin block size is the maximum amount of data that can be included in a single Bitcoin block. It is measured in bytes, and it is currently set to 1 megabyte (1MB).

The Bitcoin block size limit is a key part of what makes the Bitcoin blockchain secure and reliable. When a new block is created, the Bitcoin protocol requires miners to include a certain amount of data in the block. This data consists of the transactions that have occurred since the last block. If the block size is too small, it could limit the amount of transactions that can be included in a single block, which would lead to delays in processing and verifying transactions.

The Bitcoin block size limit also helps ensure that the network remains decentralized. If the block size was too large, it could lead to a situation where larger miners could control the network by processing larger blocks than other miners. This would lead to centralization of the network, which could be a major security concern.

In order to increase scalability, the Bitcoin Core developers recently implemented the Segregated Witness (SegWit) protocol. SegWit increases the block size limit to 2 megabytes (2MB), which is a significant improvement over the original 1MB limit. As more users and businesses continue to adopt Bitcoin, it is likely that the block size limit will need to be increased further in order to accommodate the growing number of transactions.