If you know how to use PhpMyAdmin, the following custom MySQL queries will make custom fields able to hold 65535 characters each. This change will be automatically applied to all installations in the next update:
ALTER TABLE lm_users CHANGE user1 user1 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user2 user2 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user3 user3 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user4 user4 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user5 user5 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user6 user6 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user7 user7 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user8 user8 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user9 user9 TEXT NOT NULL;
ALTER TABLE lm_users CHANGE user10 user10 TEXT NOT NULL;