Se'ouda Shelesheet:

One should be very careful to fulfill the obligation of Se'ouda Shelesheet. He should at least consume an egg-size, and if he is unable to do so, he shall not bother himself so much to do so. One who is smart shall not fill up is stomach in the morning meal, in order that he will have space for Se'ouda Shelesheet (Meqor Haim, 291:1).

The time for Se'ouda Shelesheet is the time of Minha, and if one does so before this time, he has not fulfilled his obligation (Meqor Haim, 291:2). There is no necessity to make Qiddush on Se'ouda Shelesheet, but there is a necessity to use two loaves of bread (Meqor Haim, 291:4).