The AutoNumber is a Long with the dbAutoIncrField flag set.
The matching foreign key should also be a Long.

Presumably you are creating this programmatically, so you will need
something like this:

Sub CreateTableDAO()
   Dim db As DAO.Database
   Dim tdf As DAO.TableDef
   Dim fld As DAO.Field

   Set db = CurrentDb()
   Set tdf = db.CreateTableDef("Table1")

   With tdf
       'AutoNumber: Long with the attribute set.
       Set fld = .CreateField("ContractorID", dbLong)
       fld.Attributes = dbAutoIncrField + dbFixedField
       .Fields.Append fld

       'Another example: Text field: maximum 30 characters.
       Set fld = .CreateField("Surname", dbText, 30)
       .Fields.Append fld
   End With

   db.TableDefs.Append tdf
End Sub