Doppler Frequency Resolution for Radar Doppler frequency is calculated by the formula $f_{d} = 2*v_{r}/\lambda$, where $v_{r}$ is the radial velocity and $\lambda$ is the radar wavelength. 

Normally FFT is used to detect the doppler frequency(A bank of FFT filters is used for Doppler frequency estimation).

Assume that sampling frequency is fixed as $4*f_{max}$, where $f_{max}$ is the maximum $f_{d}$ corresponding to the maximum radial velocity. 

Now I have to determine the number of FFT points to be employed in the FFT calculation. How can I determine this. 

In other words, which target parameter determines the Doppler frequency resolution.
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.