Oracle dynamic parameters error

I got the function that returns me a varchar2.

I'm using dapper and I got a problem while executing this query.

OracleDynamicParameters DictionaryParams = new OracleDynamicParameters();

DictionaryParams.Add(name: "id", value: id,direction: ParameterDirection.Input);
DictionaryParams.Add(name: "Return_Value",oracleDbType: OracleDbType.Varchar2,direction: ParameterDirection.ReturnValue);

con.Execute("function_name", DictionaryParams, commandType: CommandType.StoredProcedure);

 string a;

 a = DictionaryParams.Get<string>("Return_Value");

And I got an Oracle error:

ORA-06502: PL/SQL: error number or value :character string buffer too small

I tried to give a size to a returning value but this didn't work, and I also tried to figure out but everything didn't worked.

Can someone take a look at this?