There *are* limitations in SQL statement size, as well as inside the UNIX shell, *and* in the arsdoc command itself... You can change the maximum SQL statement size in DB2, and the shell 'environment' size in AIX. (Older versions of AIX needed a reboot, but I'm not sure that it's required anymore.)
As for arsdoc, I think the limit is hard coded, and the only solution is to reduce the size of the SQL command you're trying to pass to the database.
-JD.