Block Size

Block size refers to the maximum amount of data, in bytes, that a blockchain block can contain. This limit determines how many transactions can be included in each block, as miners cannot exceed the allowed size when assembling new blocks.

Setting a cap on block size prevents the blockchain from growing too quickly, which helps keep it accessible for regular users who want to run a full node or download the entire chain. Because the number of transactions that fit in a block is limited, miners prioritize transactions that offer higher fees—usually calculated per byte of data.

In Bitcoin, the block size was originally fixed at 1 megabyte. With the introduction of Segregated Witness (SegWit), blocks are now measured using "block weight" instead of strictly by size, allowing for an effective maximum of up to 4 MB per block, depending on transaction types.