code: func (msg *MsgBlock) SerializeSize() int {
  	// Block header bytes + Serialized varint size for the number of
  	// transactions.
  	n := blockHeaderLen + VarIntSerializeSize(uint64(len(msg.Transactions)))
  
  	for _, tx := range msg.Transactions {
  		n += tx.SerializeSize()
  	}
  
  	return n
  }
docs: // SerializeSize returns the number of bytes it would take to serialize the
  // block, factoring in any witness data within transaction.