From 6 cl to a 23 cl bowl
The capacity follows the drink. For rich, sweet liqueurs such as amaretto, Grand Marnier or a home-made nut liqueur, 6 to 8 cl is ample, because you fill it a third at most. Limoncello that comes ice-cold from the freezer asks for the same size, ideally with a thick base that holds the chill. With port and sherry you move up a size to 10 to 12 cl, because those fortified wines are poured in larger measures and need more air to open up.
The shape decides how much you smell. A tulip-shaped glass that narrows slightly towards the top, like the 8.1 cl grappa model, concentrates the vapour; a straight 8 cl cordial glass does not, and lets the alcohol evaporate faster, which is welcome with liqueurs that smell sharp. If you want one glass that can do everything, choose a small stemmed bowl of 12 to 17 cl. Port works in it just as well as rum, and your hand does not warm the drink.