witnesssize

Witnesssize is a measurement used in Segregated Witness (SegWit) transactions to quantify the size of the witness data. In SegWit-enabled networks, transaction data is split into a base part and a witness part. The witness part contains signatures, public keys, and other script data required to authorize inputs, and is stored separately from the base transaction data. Witnesssize refers to the total number of bytes of this witness data in the serialized transaction.

Witness data, together with the base transaction, determines the transaction weight. The network fee is calculated in terms of virtual size, which derives from the transaction weight. Specifically, weight = base_size * 4 + witness_size, and vsize = (weight + 3) // 4. If a transaction has no witness data, witnesssize is zero and vsize equals the base size.

In practice, witnesssize affects fees and propagation. Larger witness data increases vsize and thus the fee. SegWit also enables certain capacity economies by differentiating how witness data contributes to weight, improving efficiency for transactions with smaller base data but significant witness data.

In summary, witnesssize is the byte-length of the witness portion of a SegWit transaction, and it directly influences transaction weight and virtual size, impacting fees and block-space efficiency. See also Segregated Witness, BIP-141, transaction weight, and virtual size.