set default datatype = char (instead of varchar)
The default column data type in the SQL Server Management Studio table designer is not currently configurable.  If this feature is important to you, consider submitting a suggestion on Connect.  Be sure to specify why the feature has value to you and your organization (e.g. enhanced productivity).

The default type is configurable in the Visual Studio IDE, though.  In VS 2015, the option is available under Options-->Database Tools-->Table and Database Designers-->Column Options.