ODP.NET

Hello

I'm getting a error calling a function with ODP.NET from C#

Code:
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OracleCommand oraCom = new OracleCommand("NORMALIZACION.nif",oraCon);
oraCom.CommandType = CommandType.StoredProcedure;

OracleParameter param1 = new OracleParameter("numnif",OracleDbType.Varchar2);
param1.Value= "73667866A";
param1.Direction = ParameterDirection.Input;

OracleParameter param2 = new OracleParameter("nif",OracleDbType.Varchar2);
param2.Size = 10; //FIXME line
param2.Direction = ParameterDirection.ReturnValue;

oraCom.Parameters.Add(param1);
oraCom.Parameters.Add(param2);

oraCom.ExecuteNonQuery();

nif_norm = oraCom.Parameters["nif"].Value.ToString();

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if i write the FIXME line i get a error (ORA-06502) complaining about the size, no matter the value i wrote.
If i don't write the FIXME line, it works but nif_norm is always empty, although the function i call is a single return 'Hello';

Where am I wrong??
Any help, examples with varchar2 as return value???

BTW: the same code with the MS provider for Oracle works fine.