maybe the ddl created by hibernate limits this to 255 characters. Look at the hibernate site and see if there is a known issue with these kinds of things.
I looked into, but could not find much. Strange thing is that the other columns with a length of 4000 were created properly. As a workaround I will run the following patch on the Postgresql DB for those interested:
ALTER TABLE jbpm_variableinstance ALTER stringvalue_ TYPE varchar(4000);