what is XBee S2C zigbee mesh maximum payload size?

For transparent mode it is 84 bytes. For API mode, you can send up to 255 bytes in the Data payload. Just note that the packet will be broken up and transmitted as four separate packets.

Does that mean that one packet can only send a maximum payload size of 63 bytes?

no that would mean that your data payload can be up to 84 bytes in size.