Check your Flat File Connection Manager, make sure you've specified the appropriate data types and lengths for the fields you are mapping into Excel.

If you've not done it this way and in your Data Flow task are directly mapping your SQL Server data into an Excel Destination, you may need to add a Data Conversion task to convert the data types (you could receive an error stating you cannot convert between unicode and non-unicode string data types)

Just a shot in the dark, but hard to tell without seeing your project...