code: func (t *TxIn) SerializeSize() int {
  	// Outpoint Hash 32 bytes + Outpoint Index 4 bytes + Sequence 4 bytes +
  	// serialized varint size for the length of SignatureScript +
  	// SignatureScript bytes.
  	return 40 + VarIntSerializeSize(uint64(len(t.SignatureScript))) +
  		len(t.SignatureScript)
  }
docs: // SerializeSize returns the number of bytes it would take to serialize the
  // the transaction input.