> An index is limited to a maximum width of 253 characters adding all the
> field widths in specified in the index

I wonder why everybody say the limit is 253, when it is
in fact 252 (try it yourself).

To verify keysize, you can play with this Index key size calculator:
http://www.volny.cz/iprenosil/interbase/ip_ib_indexcalculator.htm

Ivan