Better bottles have their own sized stopper. When I picked mine up the other day, the size reminded me of the "all in one" stoppers that are not solid but had the hole in the middle for the airlock and an outside wall that could adjust in size to the neck size of the carboy. The outside diameter was 2 3/8" at the top...
Hope this helps
Fermenting: Grand Cru, Belgian Dubbel, Nut Brown Ale, Saison Sans Soul
Kegged: Honey Amber Ale, Centennial Blonde Ale,
Gone but not forgotten: Irish Red