Per RFC 1558, there is no real limitation to an ldap search filter. Some implementations may have their own upper limits - MS AD, Novell eDirectory, SunOne - you will need to inquire with the respective vendor if a limit has been put in place and what the best practices are for creating a search filter.
This field is just passing the search query directly to the directory server. It is best practice to test the filters specified for the EEM LDAP search in a utility outside of EEM to make sure it will run properly. Your directory admin will be the best resource to assist with this testing.