Karpas

(3) [Then] they set [food] before him. He dips the lettuce before he reaches the course following the [unleavened] bread. [Then] they set before him unleavened bread, lettuce, and a mixture of apples, nuts, and wine, and two dishes, although the mixture of apples, nuts, and wine is not compulsory. Rabbi Eliezer bar Tzadok says: It is compulsory. And in the Temple they used to bring before him the body of the Passover-offering.

Less than a kezayit - because with a kezayit there is a doubt as to whether an after bracha must be made or not, therefore it is better to eat less than a kezayit so there will not be an obligation according to everyone.

(2) Begin with the blessing "who brought forth from the ground" and take a vegetable and dip it in charoset and eat a kazait, he and everyone reclining with him should not eat less than a kazait. Afterwards, take away the table from in front of him and he reads the hagaddah to himself. Mix the second cup, and here the son asks. Why is this night different from all other night? On all other nights we don't dip even once, tonight twice. On all other nights we eat hametz and matzah, tonight only matzah. On all other nights we eat meat roasted, boiled, or cooked, tonight only roasted. On all other nights we eat all kinds of vegetables, tonight only maror. On all other nights we eat while sitting or reclining, tonight we all recline.