Gemara

34b (Mishnah): One may eat a Shelishi (third degree Tum'ah) with Terumah mixed in.

Question (Rav Hamnuna): If one who eats a Shelishi becomes a Shelishi, why is this permitted?

Answer: Less than a k'Zayis of Terumah is mixed with the volume of half a loaf.

Berachos 38b (R. Chiya bar Aba): I saw R. Yochanan eat a salted (i.e. pickled) olive. He blessed before and after eating it.

Question: Granted, if Shelakos (cooked foods) retain their blessing; he blessed Borei Pri ha'Etz beforehand, and me'Ein Shalosh afterwards. However, if Shelakos lose their blessing, he blessed sheha'Kol beforehand, but what did he bless afterwards?!

Answer: Perhaps he blessed Borei Nefashos!

Question (R. Yirmeyah): How could R. Yochanan bless after eating a salted olive? Since the pit was removed, it is less than a k'Zayis!

Answer (R. Zeira): It suffices to eat the volume of an average olive to say a Berachah Acharonah. R. Yochanan ate a big olive, without the pit it was the volume of an average olive.

45a (Mishnah): One must eat a k'Zayis to be (obligated in Birkas ha'Mazon and) join for a Zimun;

R. Yehudah says, he must eat a k'Beitzah.

49b (Abaye): They argue about how to expound "v'Achalta v'Savata u'Verachta." R. Meir (the first Tana) holds that "v'Achalta" refers to eating, and "v'Savata" alludes to drinking. "Eating" normally refers to a k'Zayis;

R. Yehudah holds that "v'Achalta v'Savata" connotes eating that gives satiation, i.e. a k'Beitzah.

Sukah 26a (Beraisa): R. Tzadok was given (bread) less than k'Beitzah to eat. He took it in a cloth and ate it outside the Sukah; and he did not bless afterwards.

Rishonim

Rif (Berachos 27a): We learn from the episode with R. Yochanan that for less than a k'Zayis one blesses beforehand, but not afterwards.

Rambam (Hilchos Berachos 3:12): If one ate less than a k'Zayis of bread or other food, or drank less than a Revi'is of wine or other drinks, he blesses beforehand, but he does not bless afterwards at all.

Rosh (Berachos 6:16): It is not clear whether there is a Shi'ur for Borei Nefashos. Perhaps a Shi'ur is only for Berachah me'Ein Shalosh, (which is in place of Birkas ha'Mazon,) in which it says "you will eat and be satiated and bless." It requires a Shi'ur of eating (a k'Zayis) or satiation (k'Beitzah), according to the respective opinions. Borei Nefashos does not require a Shi'ur. The question why R. Yochanan blessed afterwards was according to R. Chiya bar Aba, who says that he blessed on it me'Ein Shalosh. Or, perhaps Borei Nefashos, which is mid'Rabanan, was enacted like me'Ein Shalosh, which is mid'Oraisa. There is no proof from R. Tzadok, who ate less than k'Beitzah and did not bless afterwards. Perhaps he did not bless Birkas ha'Mazon, but he blessed Borei Nefashos. The Ri connotes that if one ate less than a Shi'ur of the seven species, he blesses Borei Nefashos. I disagree. It is unreasonable to say that if one ate less than the Shi'ur for a Berachah Acharonah, he makes a different Berachah Acharonah! Also, it seems that R. Tdid not bless afterwards at all. If he did, the Gemara would have said which Berachah he made! Also R. Yonah said so.

Rosh (7:24): It seems that one must bless for drinking even less than Malei Lugmav (the amount that one can hold in his mouth when one cheek sticks out). This is the Shi'ur for liability on Yom Kipur, but for a Berachah we do not need so much. R. Yitzchak was unsure whether a k'Zayis is enough for drinking, like for eating. Or, perhaps since R. Meir expounds that "v'Savata" alludes to drinking, a k'Beitzah is required, like R. Yehudah expounds this to require a k'Beitzah of bread (for Birkas ha'Mazon), for less than this does not satiate. Or, perhaps satiation of drinking is unlike satiation of eating. Therefore, it is good to be careful to drink only less than a k'Zayis or (at least) a Revi'is to avoid the Safek.

Beis Yosef (DH v'Tzarich): The Rosh must hold that a Revi'is is a k'Beitzah. If it were less than a k'Beitzah, how would one avoid the Safek through drinking a Revi'is?! The Rambam holds that we do not bless for less than a Revi'is.

Bach (DH u'Mah she'Chosav Beis): This is a textual error. Pesachim 109a explicitly says that a Revi'is is one and a half Beitzim!

Rashi (35a DH d'Leika): Eating never joins to a Shi'ur, i.e. a k'Zayis of Isur, or to the Shi'ur of a date for Yom Kipur, if he delayed from the beginning to the end more than Kdei Achilas Pras. The Torah gave this Shi'ur of eating regarding a house with Tzara'as. If one delays there this Shi'ur of time, his clothes become Tamei.

Tosfos (Berachos 39a DH Batzar): The Ri says that since Borei Nefashos is not an esteemed Berachah, one says it even for less than a Shi'ur. This is unreasonable. Since it was enacted corresponding to Al ha'Gafen, it requires a Shi'ur, just like Al ha'Gefen.

Poskim

Shulchan Aruch (OC 190:3): It is a Safek what is the Shi'ur of wine to obligate a Berachah Acharonah, if it is a k'Zayis or a Revi'is. Therefore, it is good to be careful to drink only less than a k'Zayis or (at least) a Revi'is to avoid the Safek.

Beis Yosef (DH v'Gam): Also in a Teshuvah (4:14), the Rosh says that Rashi was unsure if the Shi'ur for a Berachah Acharonah is a k'Zayis or a Revi'is. He brought proofs for both possibilities. It seems that the text should say 'Ri' in place of 'Rashi'.

Bach (2): The Rosh had a Safek because regarding Yom Kipur, the Shi'ur for drinking is Malei Lugmav, which is the majority of a Revi'is, which is between a k'Zayis and a k'Beitzah. Therefore, for Berachah, the Shi'ur is less than for Yom Kipur, i.e. it is a k'Zayis. This is like we find regarding eating, that the Shi'ur for Yom Kipur (a large date) is larger than that of Berachah (k'Zayis). Or, perhaps since the Halachah follows R. Meir, who expounds v'Savata to teach about drinking, we require a k'Beitzah. Or perhaps satiation for Berachah is less than k'Beitzah, or it is a full Revi'is, like Beis Shamai hold regarding Yom Kipur. Perhaps Beis Hillel agrees that this is the Shi'ur for satiation for Berachah.

Taz (210:1): The Shi'ur of a Revi'is does not apply to whiskey. One cannot drink a Revi'is of it! Therefore, we follow the Shi'ur of what most people drink of this. A proof is from what the Beis Yosef brings from R. Yonah that if a whole Briyah (creation) is normally eaten with the pit, it is considered like a k'Zayis with the pit. Here also, it depends on how people normally drink. In Shabbos 77a, we say that one is liable for taking into Reshus ha'Rabim the amount of wine needed to mix a cup, i.e. a quarter Revi'is, for when mixed with water it will be a Revi'is. Rava says that even though it is not a Revi'is, it is important, for one can mix water with it and get a Revi'is. Likewise, we measure whiskey based on how much people drink at once or in two times without pausing in between the time to drink a Revi'is, for this is the time to join. Since it could be mixed to make a Revi'is, and normal people cannot drink a Revi'is (undiluted), this is called "v'Savata". Also, there is a side to say that a Revi'is is not needed for Borei Nefashos. Tosfos was unsure about this. Therefore, we can rely to say that it is as if he drank a Revi'is, and he must bless afterwards.

Rebuttal (Magen Avraham 4): The same Safek applies to other drinks. Also for strong wines, and good foods such as jam, a small amount satiates, yet Chachamim did not give a different Shi'ur for a Berachah Acharonah for them.

Har Tzvi (1:159): The Rambam holds that just like eating never joins over a time more than what is normal to eat a half-loaf, drinking never joins over a time more than what is normal to drink a Revi'is. The Taz holds that since we expound "v'Savata" to teach about drinking, the Shi'ur is whatever satiates.

Shulchan Aruch (ibid.): One cannot drink less than a Revi'is of the Kos Shel Berachah, for whenever a Kos is needed one must drink Malei Lugmav, which is the majority of a Revi'is. Therefore, he should drink a full Revi'is.

Mishnah Berurah (13): It is most of a Revi'is for an average person. A bigger person must drink more. In any case one need not drink more than a Revi'is.

Mishnah Berurah (14): The Taz says that letter of the law, Malei Lugmav suffices, so one blesses after this amount, just l'Chatchilah one should drink a full Revi'is to fulfill all opinions. All the Acharonim rejected this. One may not bless on less than a Revi'is, even for strong drinks.

Shulchan Aruch (210:1): One who eats less than a k'Zayis or drinks less than a Revi'is of wine or other drinks blesses beforehand, but not afterwards. Some are unsure whether we say a Berachah Acharonah on wine for a k'Zayis. Therefore, it is good to be careful to drink only less than a k'Zayis or (at least) a Revi'is.

Beis Yosef (DH veha'Rambam): The Rambam is sure that Borei Nefashos requires a Shi'ur, and also the Rif connotes like this.

Mishnah Berurah (1): If one paused in the middle of drinking, some say that the two times do not join. Some say that they join up to Kdei Achilas Pras (the time to eat half a loaf). The Gra holds like this. It is normal to drink hot tea or coffee, and it is hard to drink it like other drinks, without stopping. Opinions differ about whether one should say a Berachah Acharonah. The custom is not to bless. Meticulous people let a Revi'is cool off, so they can drink it without pausing. It is good to do so, to fulfill all opinions, especially since the Gra says that up to Kdei Achilas Pras joins.

Magen Avraham (5): If one drank a k'Zayis and he does not have a Revi'is to drink, he should not complete the Shi'ur of a k'Beitzah, for some obligate a Berachah Acharonah on a k'Beitzah.

Machatzis ha'Shekel: Even though he already has a Safek Berachah and he cannot bless, he should not enter another Safek, for some obligate a Berachah for a k'Beitzah.

Mishnah Berurah (12): If he drank a k'Zayis and he does not have a Revi'is, he should not even complete the Shi'ur of most of a Revi'is, and all the more so not a k'Beitzah.

Kaf ha'Chayim (190:11): If he can drink a Revi'is, he should. If not, he thinks the Berachah in his heart.

Mishnah Berurah (4): Chachamim enacted the Shi'ur for liquids to be a Revi'is, like the Shi'ur to be liable for drinking Isurim.

Kaf ha'Chayim (18): To make a Berachah Acharonah, one must drink a Revi'is in the normal time, like the law of Yom Kipur. L'Chatchilah, one should not pause in order that he can bless afterwards. If he paused, he should try to drink another Revi'is without pausing, or (if the Berachah Acharonah is Borei Nefashos) eat a food obligating him to say Borei Nefashos. If he cannot, he only thinks the Berachah, for it is a Safek.