Mining Rewards – What is the smallest possible Coinbase transaction/block size?

Every transaction, including Coinbase transactions, must have at least one input and one output. Since the output value is encoded with a fixed number of bytes, not charging a block subsidy does not make Coinbase transactions smaller.

The smallest Coinbase transaction is identical to the smallest regular transaction, except that it must include the block height as the first element of the script signature (BIP34). This also means that the size of the smallest possible block increases with block height.

So the smallest possible Coinbase transaction at the current block height is 8 (version, lock time) + 2 (number of inputs and outputs) + 40 (input dictionary output and sequence) + 5 (scriptsig with size prefix) + 8 (output values ) + 1. (empty scriptpubkey) = 64 bytes long.

Adding a header of 81 bytes makes the smallest possible block length at the current block height 145 bytes.