Problems with transmission between ATtiny85 and Arduino Uno with 433MHz RF modules and Manchester
You set receivedSize to zero at instantiation, then load it with buffer[0], which also appears to be zero.  You should check that value to ensure it's correct and that it's at least 2 as 1 isn't less than. Otherwise when you go into the loop, i is 1 and receivedSize is zero (i is > receivedSize.)  So it fails to enter the loop, so no write.

uint8_t receivedSize = 0;

receivedSize = buffer[0];
for (uint8_t i = 1; i &lt; receivedSize; i++)
     Serial.write(buffer[i]);


There may also be a logic problem. If you receive one byte, the receivedSize will be 1 and it still won't print.  Keep in mind that the array goes from 0 to limit and your 'data' lives starting in buffer[1].  Meaning if 6 characters are in the buffer only 5 (1 thru 5) will print. It may be much more clear to add the index at the position 'buffer[i + 1]' then the i can run it's normal 0 to n.