Hi Olivier,
In the example shown Accelerate the Discrete Fourier Transform with NXP SPT the number of points used for Range FFT is M=512 and for Doppler FFT is N=256, which means that one frame has N=256 chirps, and each chirp has M=512 samples.
In order to perform the FFT algorithm, the number of points (M and N) must be a power of 2. In general, the larger number the better the resolution.
Regards,
Iulian