You have to use raw_input() instead (Python 2.x), because input() is equivalent to eval(raw_input()), so it parses and evaluates your input as a valid Python expression.
while True:
s = raw_input('Enter something : ')
if s == 'quit':
break
print('Length of the string is', len(s))
print('Done')
Note:
input() doesn't catch user errors (e.g. if user inputs some invalid Python expression). raw_input() can do this, because it converts the input to a string. For futher information, read Python docs.