Block Size
In short: Block size refers to the total amount of data that can be inserted into a single block in the chain, and usually means the maximum that a blockchain's…
Block size refers to the total amount of data that can be inserted into a single block in the chain, and usually means the maximum that a blockchain's rules allow. Together with how often blocks are produced, it sets how many transactions a network can process.
Why it matters
A larger block limit fits more transactions into each block, which can ease fees when demand is high. But bigger blocks take longer to spread across the network and need more storage and bandwidth, making it harder for ordinary users to run full nodes. This trade-off between capacity and decentralization drove Bitcoin's block size debate, which led to the SegWit upgrade on Bitcoin and to Bitcoin Cash splitting off as a separate chain with larger blocks. Some networks measure capacity differently; Ethereum, for example, uses a gas limit per block rather than a fixed size in bytes.
Example
When many users send transactions at the same time, more are waiting than the next block can hold. Miners or validators usually pick the transactions that pay the highest fee for the space they use, and the rest wait in the mempool for later blocks. A user who pays a low fee during a busy period may wait through several blocks before being confirmed, while a higher fee makes quick inclusion more likely.