Oracle Import problem caused by different character sets
If that is the actual DDL you are using to create the table, you could use the NLS_LENGTH_SEMANTICS parameter.  If you set that to CHAR rather than the default of BYTE, a VARCHAR2(5) will be allocated enough space to store 5 characters in the database character set (potentially up to 20 bytes) rather than 5 bytes (which could allow just 1 character).

Unfortunately, changing the NLS_LENGTH_SEMANTICS probably won't be terribly helpful if you're relying on the import process to create the table-- the dump file will inherently add the CHAR or BYTE keyword so it would actually issue the statement

CREATE TABLE BDATA.Artikel(
    Key                   VARCHAR2(3 BYTE)  NOT NULL,
    Name                  VARCHAR2(60 BYTE) NOT NULL,
    Abkuerzung            VARCHAR2(5 BYTE)  NOT NULL
);