According to the source code:

const static unsigned int max_frame_payload_size = MAX_FRAME_SIZE-sizeof(RF24NetworkHeader);

that would be 24 bytes. Maximum lenght of payload for NRF24L01+ is 32 bytes and the RF24NetworkHeader seem to be 8 bytes long. So you can send up to 24 bytes in one package.

According to the source code:

const static unsigned int max_frame_payload_size = MAX_FRAME_SIZE-sizeof(RF24NetworkHeader);

that would be 24 bytes. Maximum lenght of payload for NRF24L01+ is 32 bytes and the RF24NetworkHeader seem to be 8 bytes long. So you can send up to 24 bytes in one package.

EDIT:
Here is the stripped RF24NetworkHeader declaration code from which can be seen sizeof(RF24NetworkHeader) is 8 bytes.

struct RF24NetworkHeader {
  uint16_t from_node;           // 2 bytes
  uint16_t to_node;             // 2 bytes
  uint16_t id;                  // 2 bytes
  unsigned char type;           // 1 byte
  unsigned char reserved;       // 1 byte
  static uint16_t next_id;      // static member doesn't count for sizeof()
  RF24NetworkHeader() {}
  RF24NetworkHeader(uint16_t _to, unsigned char _type = 0): to_node(_to), id(next_id++), type(_type) {}
  const char* toString(void) const;
};