What are the various shitot regarding the shiur of water for mayim acharonim.
The rishonim do not give a shiur for mayim acharonim, and write that one needs to wash with the amount of water than ensures one’s hands will be clean (Raavad, Tamim De’im 66; Rashba, Toras Ha-Bayis, Shaar 2; Orchos Chaim, Netilas Yadayim 14; Seder Hayom, p. 67). This is also the simple ruling among later authorities.
However, the Vilna Gaon required a full revi’is (and from a keli), as cited in Maaseh Rav (84), and explained in Teshuvos Vehanhagos (Vol. 2, no. 114). Therefore, those who follow the customs of the Gaon should use a revi’is, whereas others can rely on the lenient opinions that it is sufficient to use enough water that ensures clean hands.