There is no way to limit an attribute value length in Directory Server, other than writing a custom plugin that will intercept ADD and Modify operations.
The {x} length indication in the Syntax notation of an attribute description (in cn=schema) per X.500 specification is just an indication of the minimal MAXIMUM size that compliant products must support.
Sun Directory Server has a single (configurable) limit which is the size of the LDAP requests (nsslapd-maxbersize).
Regards,
Ludovic.