The Prophet ﷺ set the amount as one sa' of staple food. A sa' is an old measure of volume, not of weight. The foods named in the hadith include dates, barley, raisins and dried cheese. Today many people give the main food of their area, such as rice or wheat.
Because a sa' is a measure, its weight is different for each food. Local religious authorities publish an amount each year, so follow the amount given by your local authority or scholar. The Hanafi school counts half a sa' of wheat as enough, because the early Muslims used this measure.
Scholars differ on the form of payment. The Hanafi school says that you may give the money value instead of food. The Maliki, Shafi'i and Hanbali schools say that food should be given. Many charities collect money and buy food for families, which makes giving easy.