As mentioned previously, the Shulhan Aruch (Orah Haim 210:1) rules that the minimum volume required to recite an after-blessing on a beverage is a Revi’it. The Shulhan Aruch (ibid.) goes on to state that when one drinks the volume of a Kezait (roughly 28 cc.), the requirement to recite a Beracha Aharona is Halachically questionable. The Mishna Berura (O.H. 210:10) writes that this also applies to other beverages. The Kezait measurement applies to many other foods and therefore there is some doubt as to whether or not it applies to drinks as well.
When it comes to alcoholic beverages, such as Arak, the Taz (O.H. 190) is of the opinion that one would be required to recite a Beracha Aharona even if he drinks less than a Revi’it. His rationale is that such beverages are typically consumed in that amount and not in the much higher Revi’it volume. The Taz even states that Kiddush could be recited over less than a Revi’it of whiskey. Nevertheless, the custom in Morocco was to require a Beracha Aharona only after drinking a Revi’it, be it alcohol or any other type of beverage.
It should be noted that the Shulhan Aruch states that in order to avoid any doubts, one should try and either drink at least a Revi’it, or less than a Kezait, in which case no Beracha Aharona is to be recited.
Summary: The minimum amount required to recite a Beracha Aharonaover beverages is a Revi’it (see previous Halacha for the actual amount of a Revi’it). One should ideally drink either less than a Kezait, or a Revi’it or more, to avoid any doubt vis-a-vis a Beracha Aharona.