Oracle DDL's create fields with 2000 characters. String variables with less than 4000 characters are stored in this text field, only strings with length greater than 4000 characters are stored as a byte array. Oracle throws an exception when a process has a variable with a length between 2001 and 4000 characters.
See discussion: http://forums.activiti.org/content/bug-oracle-ddls
Activiti 5.16.1, Oracle 11.2