Rather than fixing the size, use one of the first few bytes to
indicate how wide the sector numbers are. Since you only need to
remember the log base 2 of the size, a single byte can make it
possible to deal with up to 2^255 bits, or quite a bit more sectors
than anybody is likely to ever need.
32 would probably be fine for now. (Why hasn't Microsoft ever come out
with a 32-bit FAT filesystem? They still have 16-bit cluster numbers,
which is why the cluster sizes are so huge.)
-- - David A. Holland | Average number of times an American dholland@hcs.harvard.edu | opens the refrigerator each day: 22