One Bitcoin Block contains all the data needed to verify the transaction and make it possible to trace the previous block in the Blockchain.
The size of a block simply is how many bytes that one block contains, which is shaped by the number of transactions.
You could have a look at the following table to see the sizes of some block data.
The block header is the list of meta data about the block, which is an 80-byte field. The next pieces of information are the number of transactions in the block and the transactions themselves.
Regularly, one transaction is above 250-byte depending on the type of that transaction, multi-signature transactions need much more space than single-signature transactions because of the required conditions on spending. Averagely, one block contains about 500 transactions, which is less than a transaction per second.