Doppler Frequency Resolution for Radar
Doppler resolution usually depends on the dwell time $T_{dwell}$, i.e., the time for which the radar is going to stare (look) at the target. If the pulse/sweep repetition interval is given by $T_{PRI}$ then $N = T_{dwell}/T_{PRI}$ is the number of pulses/sweeps in one dwell, which is also the limit of your FFT size. Your maximum Doppler corresponds to $N/2$th bin, therefore the Doppler resolution is $f_d/(N/2)$. Note that more pulses you are able to send to the target, better is the Doppler resolution.