> Which is the maximum size for an string index?

252 for single column index on single byte charset with binary collation.
84 for single column index on single byte charset with non-binary collation
or multi-byte charset.
Approximately 20% less for multi-column indexes.

You can verify some combinations using this calculator:
http://www.volny.cz/iprenosil/interbase/ip_ib_indexcalculator.htm

Ivan