The create/schema/gen.pl that generates the SQL statements for Oracle sets the nvarchar2 fields to 2,048, which is very high.
It failed with the default settings on Oracle 11.07 and is very likely to be a problem at many sites.
The size required depends on the locale (i.e. character set), but can hopefully be less than 2,048. We got away by setting it to 2,000.
I'll attach a patch that resolved our problems.