Here I wrote a small script that you can put into a script element and which evaluates to the remaining bytes:
~~~~~~~~~~~~~8<~~~~~~~~~~~~~
# get collection with results so far
results = currentMapper.getCurrentResults()
# get latest added result
lastResult = results.getLastResult()
# access the parsed value
value = lastResult.getValue()
# get the value parsed
stringLength = value.getUnsignedNumber()
currentPos = currentMapper.getCurrentOffset()
remainingBytes = currentMapper.getCurrentRemainingSize()
actualSize = min(stringLength, remainingBytes)
byteView = currentMapper.getCurrentByteView()
string = byteView.readString(currentPos, actualSize, "ISO-8859-1")
stringValue = Value()
stringValue.setString(string)
currentElement = currentMapper.getCurrentElement()
results.addElement(currentElement, actualSize, 0, stringValue)
~~~~~~~~~~~~~8<~~~~~~~~~~~~~