Truncating the table will not change it's definition, and disabling constraints is also not necessary, the error relates to the fact that it is trying to insert data which is outsides the bounds of the coloumn.

eg.

If you have userA and userB both with a table CUSTOMER but userA has the NAME column defined as varchar2(20) and userB has the NAME column defined as varchar2(10). If the names contained in userA's table are greater than 10 characters in length if you try to import the table data into userB the import will fail with the error you are getting.

Either drop the tables in the user you importing into or update the table defintions to match the user you are exporting from.

HTH