How to obtain the Spectrogram of a signal
Since the input signal is real, half of the FFT data is "useless" because it is simply the complex conjugate of the other half. More precisely, a 256-long FFT of a real signal will give you: the DC amplitude, 127 amplitudes, the amplitude at Nyquist, and 127 conjugate amplitudes.

Among the 700 chunks, 256 / 32 - 1 = 7 extend outside the boundaries of the original signal.

Hence, 129x693.