The memory is resized, but the value returned by len() doesn't change:

>>> b = ctypes.create_string_buffer(23)
>>> len(b)
23
>>> b.raw = '0' * 23
>>> b.raw = '0' * 24
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: string too long

>>> ctypes.resize(b, 28)
>>> len(b)
23
>>> b.raw = '0' * 28
>>> b.raw = '0' * 29
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: string too long