fft length

on what basis the fft length is calculated related to the sampling
frequency?I want to search for a maximum peak by obtaining the spectrum
of
the signal.I am working on demodulator in that my task is to acquire the
carrier from the receive modulated signal.Modulation used is BPSK.It can
be
used for 300,600,1200,2400,10800,5400 bit rates.sampling frequency is
518400 for 10800 and 5400 bit rates,for remining cases Fs is 460800.
can any one tell me how to decide the fft lenght depending on bit
rate.can
i use same length fft for all bit rate?
Thans in advance.
Regards,
lakshmi.