The Torah requires that when one fulfills a Commandment one must fulfill it to a significant degree. It is not sufficient, for example, to merely sip the wine. One must drink a significant amount. The Torah has therefore mandated minimum amounts for all commandments.
Each wine cup for the Four Cups must hold a minimum of four and a half ounces. A sick person may be more lenient and use a cup as small as three ounces.
When drinking the Four Cups, one must drink a «majority» of the wine, i.e., more than half the wine that is in the cup.
When eating Matzah and Koraich, one must eat an amount that covers an area of at least seven by six-and-a-half inches.
If one uses romaine lettuce for the Bitter Herbs and Koraich, one must eat an amount that covers an area of at least eight by ten inches. That usually necessitates eating two or three leaves.