How Much Matza To Eat

Motzi, Matza
For motzi and matza together, you must eat at least 1.9 fl. oz. (56 ml) of matza within a 4-minute period from when you begin eating.

Afikoman
For afikoman, you must eat another 1.9 fl. oz of matzab'di'avad, at least 1.3 fl. oz. (39 ml, or 1/6 cup) is sufficient. 
Note This amount is about 1/2 of a machine shmura matza, or 1/3 of a hand shmura matza
Note If your mouth is too dry to eat that quickly, you may drink water with the matza.