The block size is the combination of the block header and the list of transactions. Specifically, the block header has these fields:

• version - 4 bytes
• previous block header hash - 32 bytes
• merkle root hash - 32 bytes
• time - 4 bytes
• nBits (encoded POW target) - 4 bytes
• nonce - 4 bytes

So the total bytes for a block header is 4 + 32 + 32 + 4 + 4 + 4 = 80 bytes

After the block header is the list of transactions which is

• transaction count - 1-9 bytes
• transactions - varies

Transactions can vary greatly in terms of the number of bytes, but the general serialization of a single transaction is:

• version - 4 bytes
• tx in size - 1-9 bytes
• tx ins - see below
• tx out size - 1-9 bytes
• tx outs - see below
• locktime - 4 bytes

Each tx in has these fields:

• previous tx out hash - 32 bytes
• previous tx out position - 4 bytes
• pubkey script - varies depending on script, max 10,000 bytes
• signature script - varies
• sequence - 4 bytes

Each tx out has these fields:

• value - 8 bytes
• pubkey script length - 1-9 bytes
• pubkey script - varies

The sum total of all these things in a block determine the size, which currently has to be less than 1 MB.