Here is a sample test case that will help you to undestand this:

Code:
Where 50050 is the LATIN CAPITAL LETTER A WITH CIRCUMFLEX
      14844588  is the EURO SIGN

    
SQL> create table test (testcol varchar2(20));
SQL> create table test1( testcol nvarchar2(20));

SQL> insert into test values (chr(50050));
SQL> insert into test values (chr(14844588));

SQL> insert into test1 values (nchr(50050));
SQL> insert into test1 values (nchr(14844588));

SQL> commit;

SQL> select dump(testcol) from test;

DUMP(TESTCOL)
--------------------------------------------------------------------------------
Typ=1 Len=2: 195,130
Typ=1 Len=3: 226,130,172

SQL> select dump(testcol) from test1;

DUMP(TESTCOL)
--------------------------------------------------------------------------------
Typ=1 Len=2: 195,130
Typ=1 Len=3: 226,130,172

SQL> select dump(testcol,1016) from test; 

DUMP(TESTCOL,1016)
--------------------------------------------------------------------------------
Typ=1 Len=2 CharacterSet=UTF8: c3,82
Typ=1 Len=3 CharacterSet=UTF8: e2,82,ac

SQL>  select dump(testcol,1016) from test1;

DUMP(TESTCOL,1016)
--------------------------------------------------------------------------------
Typ=1 Len=2 CharacterSet=UTF8: c3,82
Typ=1 Len=3 CharacterSet=UTF8: e2,82,ac
The Length will specify the # of Bytes it takes to store in the database. Now you can make your own judgement call

-Sam