8. It is forbidden to read the Megillah at night before the appearance of
three stars ("Tzeis HaKochavim"), even though one is suffering great
discomfort due to the fast [of Esther]. However, one may consume a little
(1) before the Megillah reading, such as coffee and similar beverages, in
order to strengthen oneself from the exhaustion of the fast.
(1) One may eat the volume of a "k'beitzah" (lit: "like an egg") of bread
and cake, or liquid in the same quantity (Mishna Berura 692:14). Opinions
as to the size of a "k'beitzah" range from 86.23 ml to 100 ml. In more
concrete terms, a "k'beitzah" is two thirds of a slice of white bread or
three quarters of a square matzah (Halachos of K'zayis by Rabbi Y.