Wimax FFT size support question

I have been looking into the 802.16 Wimax system ( OFDMA ) to get a
basic understanding. The system seems to be able to support multiple
FFT sizes, such as 128, 256, 512, 1024 etc. I am not able to
understand couple of issues:

1. Is the FFT size fixed for a basestation(BS)? If not when can it
change? How would a system designer go about selecting the FFT size?
2. Does a "compliant" subscriber station (SS) have to suport all
possible FFT sizes? How does an SS know what is the FFT size to use?
Presumably it has to do an FFT before getting any information from the
BS. Does it scan using different sizes at power up?
3. Is there a "common practice" FFT size? If most BS are using a
smaller FFT size, designing SS with the processing capacity to handle
the maximum FFT size is not optimal.

Thanks.

-Dipu