How much wine is one required to drink by Kiddush?
The person who makes Kiddush is required to drink enough wine to fill his cheeks. This measurement is referred to as Malei Lugmav. This does not refer to the amount of liquid that fills the entire mouth, both cheeks, but rather the amount of liquid that fills a single cheek, in a way that when the liquid is brought to one side of the mouth, it appears that his cheek is full. In an average size mouth, this volume of wine amounts to the majority of a Revius. Nonetheless, each person must follow his own personal mouth and cheek size, and must drink enough wine that his cheek becomes full in one side. In all cases, it is not necessary to drink more than a Revius of wine, even if one’s mouth is so large that a Revius does not even fill an entire cheek side [i.e. Malei Lugmav], [and in such a case a Revius suffices].
Must one drink majority of the cup if it holds more than a Revius? During Kiddush of Shabbos and all Yomim Tovim, other than the Seder night of Pesach, one is not required to drink majority of the Kiddush cup so long as he drinks the above measurement of Malei Lugmav. Thus, even if the Kiddush cup holds a number of Reviusim, and the drinking of Malei Lugmav, which on average is the majority of a Revius, will only consume a minute portion of the wine in the cup, nevertheless he fulfills his obligation and is not required to drink any more. This applies according to all opinions and may be done even initially [i.e. Lechatchila]. However, on the Seder night of Pesach there exists a dispute in this matter and although the main ruling follows the lenient opinion, and so is the custom, nevertheless, it is proper to suspect for the stringent opinion. [Despite the above, some are accustomed to always drink majority of the cup, even if it contains much more than a Revius, and one hence ends up drinking much more than a Revius. Some Chabad Rabbanim defend this custom while others deny it.]
One who makes Kiddush must drink enough wine to fill one side of his cheek, which for an average size mouth is majority of a Revius. It is not necessary to drink more than this amount even if it only consumes minority of the wine in the cup, such as by a large cup that holds more than a Revius, [however some are accustomed to do so].
If one has a very small mouth, such as a child, does it suffice to drink even less than the majority of a Revius?
Yes. Some Poskim however question this matter, and suggest that perhaps in all cases one must drink at least the majority of a Revius. Practically, by a child below Bar/Bas Mitzvah, one may be lenient.
 Admur 271:24; See also 190:4; Michaber 271:13; Rav Huna Pesachim 107a; Ketzos Hashulchan 79:5
Other opinions: Some Poskim rule one is initially to be stringent to drink an entire Revius in all cases. [Elya Raba 271:28 based on Sefer Hamanhig; Tosefes Shabbos 271:36; Yaavetz in Mor Uketzia 190; Ben Ish Chaiy Bereishis 2:23; Kaf Hachaim 271:83; Menorah Hatehora 271:24; Likkutei Maharich based on many Geonim; See Piskeiy Teshuvos 271 footnote 331]
 Admur ibid; Tosafus Pesachim ibid and Yuma 80a
Other opinions: Some Rishonim rule that this amounts to much more than the majority of a Revius, although slightly less than a full Revius. [Biur Halacha 271:13 “Vehu Rubo”] Some Poskim rule it means exactly a Revius. [Likkutei Maharich based on many Geonim]
 Admur ibid and 190:4 and 612:12; M”A 271:29; Tosafus ibid; M”B 271:683
 Admur ibid; M”A ibid; Bach 271; M”B ibid
 Admur 472:19
 Admur ibid; M”B 472:33; Divrei Yatziv 1:135; Chidushei Hagriz on Rambam Chameitz 7:9; Chazon Ish 39:15; Piskeiy Teshuvos 271:33
 Some Poskim rule that one may even initially drink the majority of a Revius even if the cup holds many Reviusim. [second opinion in Admur ibid; first opinion in Michaber 472:9; Tur; Peri Chadash] Other Poskim however rule one must initially drink the entire cup, and at the very least majority of the cup, otherwise one is not Yotzei. [first opinion in Admur ibid; second opinion in Michaber ibid; Ramban brought in Orchos Chaim Seder Leil Pesach 6; Bach 472]
 Admur ibid; Chok Yaakov 472:10
 It was evident by Farbrengens that the Rebbe was accustomed to always drink majority of the cup.
Story of the Alter Rebbe: The Rebbe Rayatz retold the following story that occurred with the Alter Rebbe: When the Alter Rebbe lived by his in-laws in Vitebsk, he and his wife were ostracized by his in-laws due to his habitual fasts and acts of penitence, and learning and Davening extensively, which did not find favor in their eyes. One Shabbos evening upon arriving to his in-laws at a late hour due to his extensive prayer, he could not find any wine or cup to make Kiddush, as they were all put away and locked by his mother in-law. Admur was then brought a heavy alcoholic beverage by his sister in-law, and poured it into a large bucket, being the only vessel he could find. Admur drank majority of the content of the vessel, and due to his small body weight and having fasted the entire day he entered into great danger. At last he found some leftover food and used it to enliven his soul. [Sefer Hasichos 1937 p. 193; Kerem Chabad 4:10] It is due to this that the Alter Rebbe was accustomed to eat dairy porridge every Motzei Shabbos in memory of the miracle. [Kerem Chabad ibid] Some derive from the above story that one should drink majority of the cup in all cases. In Kerem Chabad they write “he drank majority of the cup as is required by Halacha”.
 Rav Avraham Elyashvili; Rav Asher Lemel Hakohen; Rav Y.L. Groner in a correspondence related to me that so is the custom of Chassidim
 Rav Ginzberg in Hiskashrus [States one cannot derive anything based on a story]; Rav Eli Landa Shlita
 Derech Hachaim, brought in Biur Halacha 271:13 “Vehu Rubo”; Implication of Admur 190:4 “Malei Lugmav of the child, a small child in accordance to his small size” and Poskim ibid; Chazon Ish 39:15
 Biur Halacha ibid; Birchas Habayis 46:33
 Biur Halacha ibid