What is the maximum number of characters I can send with LMiC?

Thank a lot for your reply.
Sure, I will change my message and make sure , it is byte and not a human-readable value but I atill confuse about the convertion of my double value or char value to a bits/byte

But, the thing which still troube me. If I print

Serial.println(mydata)

my terminal return me an error.

exit status 1
call of overloaded ‘sprintln(uint8_t [64], int)’ is ambiguous

However, if I print

Serial.println((char *)mydata);

it print me an human-raedable value.

So does mydata value are already in bytes (no human-redable)? because (char *) should convert in readable value, no?
Morever, mydata is declare in this way

uint8_t mydataAll[64];

But for sure, I have to review this part of my code (and remove the ’ {}:" ’ which consist of a json format

Tnank a lot