Window length in matplotlib's specgram In the paper about speaker verification I can read that  

Two quantities have
to be set: the length of the window and the shift between two
consecutive windows. For the length of the window, two values are most often used: 20 milliseconds and 30 milliseconds. [...] For the delay, the
value is chosen in order to have an overlap between two consecutive windows; 10 milliseconds is very often used.  

and later  

Once an FFT algorithm has been chosen, the only parameter to fix for the FFT calculation is the number of points for
the calculation itself. This number N is usually a power of 2
which is greater than the number of points in the window,
classically 512.  

However, in the matplotlib's specgram I can only see NFFT and noverlap parameters which, according to the documentation, correspond to the second quotation.  

How can I then set the window's length or what is the default value of it?
You'd generally do the FFT without zero padding, i.e. window size == FFT length.

Having an FFT larger than the window leads to an interpolation, but doesn't add any information to the signal, and that's only useful in certain cases – maybe the paper presents one such case.