Which table in the AdventureWorks database are you trying to export data from? If it's the HumanResources.Department table, the DepartmentID column has a smallint data type. That equates to a two-byte signed integer (DT_12) Integration Services data type.
When you set up your Flat File Destination, the data type should automatically be correctly mapped. If not, use the 'Suggest Types' button or enter the data type manually.