Blocks are the smallest units in the Blockchain ledger. Blocks are essentially clumped-together bundles of transaction data that are eventually joined in a chain to form the distributed ledger. The amount of data that a single block may carry is defined as the block size.
For example, one block can only carry 1MB of data. Such restrictions were imposed to reduce the possibility of overloading the Blockchain and preventing any DoS attacks.
Overloading the network is another major setback for block sizes for Blockchain. The quicker blocks fill up with transactions, the more likely that transaction approval will take longer.