Universal stoppers cover several standard stopper sizes, due to their tapered shape. The stoppers also feature a lip, to help prevent them from getting lost in a carboy.
Small is roughly a #6-#7 Regular Stopper that should fit most Glass Carboys comfortably including 1 gallon and 1/2 gallon jugs.
Medium is roughly a #8-#9 Regular Stopper that should fit most Plastic Carboys comfortably including the Fermonster Widemouth Fermenters.
Large is roughtly a #10 Regular Stopper in size.