What is the maximum field length for the "group search filter" in EEM?
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.