Exactly same issue than : liquibase report 127 for size instead of 255.
The wrong line is in ColumnSnapshotGenerator.java line 206.
The result is already in chars since the NVARCHAR2 type implies you can only specify sizes in chars.
It doesn't require any division.
I think the safest way of solving this issue is to use the CHAR_LENGTH column instead of DATA_LENGTH for characters columns.
See : http://docs.oracle.com/cd/B19306_01/server.102/b14237/statviews_2094.htm