Basic data question

Projects Programming
14

Questions....said Roy Batty to the chinese eye designer..

 if (Serial.available())  {
        int n = Serial.readBytesUntil ('\n', s, sizeof(s));
        s [n] = 0;

Would it be ok to put a 2 in place of "sizeof(s)" ? Since I only need 2 bytes

I dont understand this line...

int n = Serial.readBytesUntil ('\n', s, sizeof(s));

Why an integer? isnt the data an array? And what is the function of that n

Thank you.