if (sizeof(RFID_tag) > 0){
Why does the size of the array matter? Shouldn't you be concerned with the amount of data IN the array?
What data IS in the array? Put the XBee away until you KNOW that the RFID is being read correctly.
Which pins are the XBee connected to, anyway? You seem to be using pins 0 and 1 for the XBee AND the RFID. You can't do that.