The documentation says that a serial buffer is 128 bytes: http://arduino.cc/en/Serial/Available
The code declares it as 16 or 64 bytes. https://github.com/arduino/Arduino/blob/master/hardware/arduino/cores/arduino/HardwareSerial.cpp
I guess it is more appropriate to fix the documentation but I can't seem to edit the page (I guess they are not a public WiKi). Can someone make a suitable fix?